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[Obituary Notice from the Proceedings of the Royal Society, B, Vol. 110]

FRANCIS DARWIN—1848-1925.

Francis Darwin, the third son of Charles Darwin, was born at Down on August 16, 1848: he died at Cambridge on September 19, 1925. In his 'Recollections' (one of the essays in "Spring-time and other Essays" (1920)) he says that he was christened at Malvern—"a fact in which I had a certain unaccountable pride. But now my only sensation is one of surprise at having been christened at all, and a wish that I had received some other name." When he was twelve years old he went to the Grammar School at Clapham kept by the Rev. Charles Pritchard, who became Savilian Professor of Astronomy at Oxford. This school was selected on account of its nearness to Down, and also because it "had the merit of giving more mathematics and science than could then be found in public schools." He was admitted to Trinity College, Cambridge, in 1866, where, in those more peaceful days, from his bedroom he heard the nightingales sing through the happy May nights. He described the teaching of biology at Cambridge as being "in a somewhat dead condition. Indeed, I hardly think it had advanced much from the state of things which existed in 1828, when my father entered Christ's College. The want of organised practical work in Zoology was perhaps a blessing in disguise: for it led me to struggle with the subject by myself. I used to get snails and slugs and dissect their dead bodies, comparing my results with books hunted up in the University Library, and this was a real bit of education." On one occasion "a thoughtful brother sent me a dead porpoise, which (to the best of my belief) I dissected, to the horror of the bedmaker, in my College rooms."

After obtaining a First Class in the Natural Sciences Tripos in 1870 he went to St. George's Hospital and in due course took the Cambridge M.B. degree. In London he "had the luck to work in the laboratory of Dr. Klein," who gave him "the first opportunity of seeing science in the making—of seeing research from the inside" and thus implanted in his mind the desire to work at science for its own sake. The chance of doing this, he says, came when his father took him as his assistant. He did not carry out his intention of becoming a practising physician: "happily for me the Fates willed otherwise." He returned from London to the home at Down and for eight years acted as secretary and assistant to his father.

On Charles Darwin's death in 1882 Francis went to live at Cambridge and devoted himself to Botany. He was elected a Fellow of Christ's, his father's College, and in 1884, having been appointed University Lecturer in Botany, he co-operated with the Reader in Botany, Dr. Vines, in teaching plant physiology. On the appointment of Dr. Vines to the Chair of Botany at Oxford Darwin became Reader and held that position until 1904. From 1892

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to 1895 he acted as Deputy to Professor Babington, who had long ceased to take any part in the modernised and to him distasteful methods of instruction. Before coming to Cambridge Darwin spent a few months in 1878 in the famous laboratory at Würzburg under Professor Sachs. In later years his relationship with Sachs came to an unhappy ending: he published a paper containing some criticisms of the master's researches. Having written to Sachs and obtained no reply, he travelled to Würzburg in order to ascertain the position of affairs. On asking Sachs why he was angry with him he received the reply: "the reason is very simple: you know nothing of Botany and you dare to criticise a man like me." In 1881 Darwin spent a short time working with Professor de Bary at Strassburg.

In 1882 he was elected Fellow of the Royal Society: he was Foreign Secretary from 1903 to 1909, and Vice-President 1907-1908. In 1909 the Honorary Degree of Doctor of Science was conferred upon him by his own University at the Darwin Centenary; he was the only Englishman so honoured. "I have been astounded and delighted," he wrote in a letter, "at a most charming letter from the Vice-Chancellor asking if I would have a Sc.D. at Cambridge. Of course I can only accept gratefully. I hope you approve—it is against the principle of no Englishman need apply." Reference may be made here to his activity in connection with a small volume presented by the Syndics of the Cambridge University Press to delegates at the Centenary celebrations, entitled "The Foundations of the Origin of Species, a Sketch written in 1842." The manuscript of this sketch is referred to by Charles Darwin in his autobiography as "a very brief abstract of my theory": it was discovered in a cupboard at Down after Mrs. Darwin's death in 1896, and was edited with an introduction by Francis Darwin for distribution at the Cambridge celebrations.

He was an Honorary Doctor of Science of Dublin, Liverpool, Sheffield, and Brussels; an Honorary Doctor of Laws of St. Andrews and an Honorary Doctor of Philosophy of Upsala and Prague. In 1908 he was President of the British Association at the Dublin meeting, an appointment which gave general satisfaction to his colleagues both on personal grounds and as a recognition of the claims of botany after a lapse of forty years. In 1912 he was awarded the Darwin Medal: in 1913 he was knighted.

Darwin was married in 1874 to Amy Ruck of Pantlludw, North Wales. She died in 1876 in giving birth to their son, Bernard Darwin. Many years later (1920) he wrote "The Story of a Childhood" (privately printed) which is made up of extracts of letters (1877-1891) written to the mother of his first wife; the story is of the early years of Bernard Darwin. This collection of excerpts is evidence of his love of home, of the sincerity of his statement that "the domestic and intimate parts of life are the most lastingly happy": and of his faculty of winning the confidence and love of young children. In 1886, with true prophetic vision, he wrote to Mrs. Ruck,

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when his son was ten years old, "l am sure Bernard will be a great swell at golf some day."

In 1883 he married Ellen Wordsworth Crofts, lecturer at Newnham College. They lived at Wychfield, which they built in the grounds of his mother's house at Cambridge, until his wife's death in 1903. In these peaceful years he did the best of his work, and her powers of encouragement and criticism greatly helped him in the writing of his father's life. They had one daughter, Frances, now the wife of Professor F. M. Cornford, of Cambridge.

In 1913 Darwin married the widow of Professor F. W. Maitland, who died in 1920.

It is fitting that the best literary production of Francis Darwin should be the "Life and Letters of Charles Darwin." As Sir Charles Sherrington said in his presidential address to the Royal Society in 1925, this book is "by common consent one of the most admirable and delightful accounts ever written of a great scientific life, the modesty and simplicity of the presentation contributing to its charm." He gave a true picture of his father's personality; his love of animals, his high sense of honour, his thought for those who served him, however humble their position, and his modesty. These are some of the many qualities inherited by Francis and illustrated in the two volumes of essays, "Rustic Sounds" (1917) and "Springtime and other Essays" (1920). As a reviewer of one of the volumes says: "a certain note of intimacy seems to run through the whole." In an essay entitled "Recollections," included in the later series, there are many delightful sketches of scenes and events which remained fresh in his memory. He recalls a general impression of "unwillingly attending divine service for many boyish years" and the "singular custom" at Down of having family prayers on Sunday only: the butler Parslow, who was "a kind friend to us all our lives" and had "what may be called a baronial nature." Darwin was a lover of music, though, as in other things and persons, his tastes were eclectic: as a performer he preferred the bassoon, the flute and the recorder, "one of the most ancient of wind instruments," which he described with other old instruments in one of his essays.

Darwin was a genuine lover of Nature: his sister, Mrs. Litchfield, spoke of him as "the only one of my father's children with a strong taste for natural history." An essay on "A Lane in the Cotswolds," written when he lived at Brookthorpe, reminds one of W. H. Hudson. The following sentence recalls the poet's words "The miracle of an earth reclad": "The great revolution that breaks out in the spring, when the store-houses of the plant pour nutriment into the numberless awakening buds, is a miracle annually repeated in the endless procession of life." In later life he amused himself by following the example of Leonard Blomefield, whose "Naturalist's Calendar" he edited in 1903, and other naturalists in noting the dates of the first blooms of wild

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plants. After giving a list of flowers seen in February and March he says: "To a lover of plants, this commonplace list will, I hope, be what a score is to a musician, and will recall to him some of the charm of the orchestra of living beauty that springtime awakens."

In general literature he was faithful to a few authors: among his favourites were Dickens, Jane Austen—in whom he took a special delight—"the power of endlessly re-reading the works of Jane Austen is the only advantage conferred by a bad memory"—Trollope, Thackeray, Walter Scott, Mrs. Gaskell, George Eliot—especially Middlemarch—and other, earlier writers.

I often recall what was probably the first occasion, now nearly fifty years ago, on which I spoke to Francis Darwin. At the end of one of his lectures I asked him a question to which there was no doubt in my mind an answer would at once be given; he answered me with a smile, "I have not the slightest idea." This was thoroughly characteristic of him: many men would have admitted their ignorance to an undergraduate less directly, if indeed at all; but he was always absolutely straightforward and made his students feel that he was a fellow learner. Neither by manner nor speech did he ever act the part of a superior person. In an address on "The Teaching of Science" he said, "The only lectures which impressed me, as an undergraduate at Cambridge, were those of the late Sir George Humphry: and his most striking words were confessions of complete ignorance about many parts of physiology." As a lecturer he made no attempt to acquire the art of impressing an audience by devices familiar to many speakers: unconsciously he made an appeal by his naturalness and simple directness. He had something to say and said it in the simplest way. On my asking him how he had enjoyed a certain public lecture, he replied in some such words as these: "The place where I lectured was separated from the rest of the large hall by a curtain: from time to time some wretched person pulled back the curtain to see what was going on, and at once retired, obviously saying to himself, "Good God! here's a lecture.''

It was a privilege to be associated with Darwin in the preparation of the "More Letters of Charles Darwin" (1903); his reminiscences of the family life at Down, descriptions of this and that "tiresome person," his keen sense of humour, were a constant joy.

The photograph accompanying this Notice was chosen because it shows Darwin with one of many successive companions (Scrubbins) from whom he was seldom separated. His dogs were treated as he treated his best human friends and he seemed to derive no little satisfaction from their companionship, possibly for the reason that they never bored him. As a quotation at the head of an essay on "Dogs and Dog Lovers" he chose Archbishop Whately's words—"The more I see of men, the more I like dogs." The dog he liked best was "an inferior Irish terrier who gave me much trouble and anxiety."

Darwin always found it very difficult to hide his feelings when in uncongenial

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company: he had strong prejudices, quickly formed, and was not reticent in expressing candid and terse opinions. Of him it was true to say:

"He hath a heart as sound as a bell, and his tongue is the Clapper, for what his heart thinks his tongue speaks."

Intolerant of any trace of artificiality and affectation, he was quick to blame and no less quick to praise. He realised more than most men the value to younger people of words of encouragement and approval; no one could give greater pleasure than he did by his outspoken words of affectionate regard, words that went to the heart because they came from the heart. His path through life was smooth: his hours of leisure were not curtailed through the necessity of adding to income by any form of drudgery. He was conscientious in the discharge of such duties as he undertook, but he did not willingly take upon himself tasks which made large demands upon his time, or brought him into contact with public business. He was generous in deed and thought to those less comfortably placed, and brightened many lives by his unfailing kindness and sympathetic encouragement. He was a man who inspired affection: his kindly blue eyes, his short calls in the middle of a morning's work in the laboratory, with apologies for dropping in with nothing particular to say, his sense of humour—humour which, as he said in speaking of Francis Galton, is "so priceless an antiseptic to sentimentality"—his exceptionally lovable personality, are memories which do not fade.

A. C. S.

Some presentation may now be made, by another hand, of the scientific work that Francis Darwin achieved. His publications, dated from 1872 to 1921, included, beside several scientific books, some sixty papers in addition to lectures and addresses. No attempt will be made to survey all these, and this notice only deals with the two main chapters of his biological activity. For each of these, a sketch of the subject is given with Darwin's particular contributions in the foreground, set against a background of the general position of that chapter of plant physiology. The two sketches are followed by a few notes mentioning some of his other larger publications.

I.—Francis Darwin's Part in the Investigation of Growth-Curvatures.

By growth-curvatures are understood those movements that growing parts of plants make in response to the directive influence of factors of the environment, such as unequal lateral illumination or unnatural relations to the direction of gravitation. Outstanding among the curvatures thus engendered

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are those labelled positive or negative geotropism and positive or negative heliotropism. From the close study of these phenomena during the last half-century there has arisen a whole new world of vital relations and theories. The earlier days of this human struggle to create a science we have now to survey.

Between 1880 and 1908 Darwin published a dozen papers and some public lectures on growth-curvatures, and gave in addition three presidential addresses to the British Association. In this way he came to be identified in England with this field of work. When he started his independent work the times were singularly apt for him, so we may begin with a brief statement of the influences, Continental and English which combined in the years round about 1881 to produce a blossoming time that justified a text-book of that date appearing with the title "The New Botany." The essential forces at work were those that created plant-physiology abroad and plant-biology at home. Both of these found a susceptible disciple in Francis Darwin.

In the years just before 1879 Sachs of Würzburg had carried out a series of studies of the anisotropic behaviour of plants—a term including the lateral curvatures by unequal growth already mentioned, as well as a variety of other aspects of unilateral organisation. First for geotropic curvature, and later for heliotropic also, he came to the conclusion that these reactions were not engendered by direct control of the external conditions but belonged to the chapter of physiology which centred in "Irritability"—the response to stimuli. He was struck by the similarity of these responses of plant-movement with the motor responses of animals, and he held that the sensitiveness of the plant, or of its parts, to one kind of stimulus in one region and to another kind in another region was a general biological principle for plants, analogous with the provision of sense-organs in animals.

Sachs was a brilliant experimenter—"the father of experimental plant physiology" and invented most of the simple methods still used for demonstrating the reality of the physiological attributes of the living plant.

The year 1881 was marked out by a great botanic event. It was then that Plant-Physiology may be said to have crystallised out in its pure form from the general solution of mixed botanical knowledge. In that year W. Pfeffer published the first issue of his masterly treatise in which the subject was given its general set of primary concepts, arranged in the clear philosophic relation that was to determine the facies and articulation of the subject for the next 50 years. In no part of his text-book was the treatment more brilliant than in his presentation of Irritability.

Turning to our own country at the same epoch, we find the outstanding figure of Charles Darwin who had been spending a succession of years in the detailed study of the biological behaviour of plants. His volumes on fertilisation and on climbing plants were followed in 1880 by one entitled "The Power

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of Movement in Plants." All this extensive work was based on the outlook and problems that arose from his previous survey of Natural Selection. For each exploration Charles Darwin studied the life of a great variety of plants and made out the agreements and differences in the domestic economy of their individual lives. Such work was a blend of biology and physiology and has been labelled ecology, earning for its inventor the style of the "father of plant ecology."

Having sketched the giant scientific forces that were then energising the study of plants we note that in 1874 a young man of 26 about to take the M.B. degree at Cambridge, started his scientific career by assisting his father with experimental work at Down, in preparation for the volume that was planned on the movements of plants. When this appeared, completed, in 1880 the title page bore the inscription "by Charles Darwin assisted by Francis Darwin." Throughout his life Francis Darwin had a profound belief in the philosophical identity of inheritance, memory, habit and the mnemonic association of stimuli. He never declared whether the parental influence came by inheritance or as an external stimulus during this apprenticeship, but his career shows that the lines of thought and of experimentation then initiated became a deep-seated habit for the rest of his life. For a short time during this period of work with his father, Francis left Down and went on a pilgrimage to the laboratory of Sachs at Würzburg where he made daily contact with that brilliant autocrat and with the current work in physiology. His scientific personality now took its final shape and when his father died in 1882 he was launched on an independent career as a talented experimental plant physiologist whose mental interest in his work was strongest on the recurrent question of what might be the biological origin and significance in the life of these plants of the particular attribute under investigation.

Having sketched the development of the worker, we pass to consider the work, and we may start by stating the general outlook on the nature of Growth-Curvatures that was coming to be adopted about 1880.

Heliotropic curvatures under lateral illumination had been regarded by De Candolle as just the differential outcome of strong light on one side and weak light on the other side, determining different local rates of growth on the sides with the inevitable result of curvature. The investigators of the period we are considering all rejected this view and insisted that the curvature was not an outcome of oppositions but a harmonious entity, a manifestation of Irritability, being a motor-reaction following the perception of a stimulus at a sensitive region, the reaction being produced by the conduction of excitation from the perceptive to the motor region, whereby the uniform growth rate on all sides became rearranged as different growth rates on the two sides. By Pfeffer it was discovered that, as long ago as 1824, Dutrochet had expressed the view that these curvatures were not forced on the plant by external conditions but

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self-generated at an indication from outside. This view, which had been quite neglected, was now actively adopted.

The Darwins regarded these curvatures as essentially biological adaptive responses to external signals arising in the illuminational or gravitational state of the environment; the response and stimulus having become linked together by association during the evolution of plants, so producing forms more fit in the struggle for existence.

In the preface to "The Power of Movement in Plants" we find Charles Darwin stressing his view that all these growth-curvatures of plants, which are so important as adaptations for existence, need not be regarded as a completely new type of growth-effect but may be derived as an extension of a pre-existing general tendency for growing plants to carry out continually circumnutatory bendings in all directions, even under uniform outside conditions. In later years, after criticism by Wiesner, Francis Darwin admitted that we still lack the knowledge to justify this view of the origin of growth-curvatures.

We now pass on from our starting point to give an outline of the progress in collecting evidence about the nature of growth-curvatures from 1878 to 1908 when Darwin made his last contribution. His twelve contributions are not referred to by title but are designated by roman numerals in order of publication and the date is given with each numeral. The lectures and addresses are distinguished by dates only.

Darwin's first contribution (I, 1880) would be the work presented in the "Movements in Plants" in association with his father. Here we find recorded the early discovery of localisation of a special region sensitive to external directive stimuli. The case was that of the localisation of the light-sensitive region in the tip of a seedling grass. When a black glass tubular cap was fitted over this tip, one-sided light ceased to bring about the normal positive heliotropic curvature. Experimental demonstration of a localised region for perception of gravity was more difficult because we have no materials that are opaque shields against gravitation. Amputation of tips seemed then the only possible test: after amputation of 1.5 mm. of the root tip no geotropic sensitivity was left and the root grew straight on, when laid horizontal. If the root had been exposed horizontally before the tip was cut off, then curvature downwards would go on developing. This seems satisfactory evidence of localisation of "gravi-perception," unless we admit that the shock of the operation had rendered the plant insensitive to gravity. Owing to this unresolved doubt the decision remained in suspense for a period.

An early independent contribution by Darwin (III, 1882) with experiments on splitting the root-tip in various ways without actual amputation, provided some support for this localisation.

A further contribution (IV, 1882) to the theory of curvatures was made by

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Darwin with his research, conducted in Sachs' laboratory and published in Germany. He examined the effect of uniform all-round light on the growth-rate of the root of White Mustard and found that light depressed this rate as compared with darkness. Now it had already been shown that the Mustard root curves away from a source of light, implying a greater growth on the side towards the light. This seemed to prove conclusively that the curvature could not be the direct outcome merely of the difference of illumination on the two sides. If the curve should have had this simple origin then the expectation would be more growth on the side away from the light and a positive heliotropic curvature.*

In 1891 Darwin gave a presidential address to the Biology Section of the British Association, in which he stressed the evidence for regarding all curvatures as examples of irritability and showed how there had been a general abandonment of the view of De Candolle and a new adoption of the neglected early view of Dutrochet (1824). The second topic in this address was a survey of views upon the mechanical cause of the observed curvature, establishing that it was due to real differences of growth on the opposed sides.

He also set out the criticism by Professor Wiesner (1881) who declined to believe that circumnutation of plants was universal and refused to regard it as the origin of heliotropism.

In 1895 came a new discovery from Germany in which Pfeffer and Czapek took part. They provided evidence that the perception of gravity really is located in the root-tip, by the use of Czapek's "glass boot." This boot is a very short piece of glass tube of a size to fit tightly on the root-tip, and bent at right angles in its middle, so that when once the root has grown into it the root-tip is kept at right angles to the general axis of the root. If the root-axis is now placed horizontal with the booted tip pointing down, the gravity excitation does not arise and growth in a horizontal direction continues indefinitely. The effects of other positions all support the same interpretation.

Darwin's next contribution (VII, 1899) aimed at getting evidence of localisation of gravi-perception for the shoot as well as the root. It was found possible to support very light grass seedlings by introducing their shoot-tips into glass tubes of exact fit. If the glass tube were fixed horizontal then excitation of the tip would persist indefinitely. It was found that seedlings thus supported carried out continuously curved growth so that a spiral or a knotted form of the shoot resulted.

In 1901 Darwin gave the evening lecture to the British Association. In this he set but the evidence obtained by the use of controlling glass tubes—-the

* It must be mentioned here that 33 years later it was pointed out by Blaauw that the side of the root away from the source of light is really more brightly lit inside owing to the root having the form of a cylindrical lens. So the evidence of this experiment of Darwin's has now lost the force that had been universally attributed to it.

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work of Pfeffer and Czapek and his own experiments of 1899. Towards the end of his lecture he spoke of "unconscious memory" and the wide conception of memory formulated independently by Butler and by Hering, and expressed his inclination to interpret on these lines the acquisition of the power of curvature by plants in the course of evolution. He would class animals and plants together from a psychological point of view.

Just before this lecture was given, there came from the Continent a new conception: the "statolith theory" brought forward in 1900 by Nemec and by Haberlandt. These physiologists and also Noll urged the view that the "falling starch grains" known to occur in root-tips and in the endodermis of stems are the intermediaries between the direction of gravity and the excitation of protoplasm. With any change of position of the plant tissues these grains can be proved to fall to the lowest part of the cell that contains them and lie there pressing on the protoplasm at some quite unaccustomed region. The cells containing falling starch are to be regarded then as sense-organs and the protoplasm of their upper, lower and radial walls must all have different excitabilities so that the appropriate movement is initiated to bring the cell into its normal position with the starch lying on the original bottom of the cell.

To this theory of gravi-perception Darwin gave immediate adherence and carried out three short investigations which supported it. In contribution (IX, 1903), he showed that the stimulating effect of starch grains on an unaccustomed side of the cell is heightened if the tissue is kept in vibration on the prong of a tuning fork so that the starch grains dance on the protoplasm surface. A later contribution (X, 1904) dealt with the exposure of roots and shoots to weak centrifugal force which had long been known to excite in the same way as gravity. He found that with a force sufficient to displace the starch grains the appropriate geotropic excitation occurs.

In his next contribution (XI, 1904), he dealt with the problem of the similar occurrence of falling starch throughout a typical root system, made up of a primary root bearing secondary roots, which again bear tertiary roots. Bach of these three classes has its own type of directional reaction to gravity, the primary being positively geotropic, the secondaries diageotropic, while the tertiaries show no directional control by gravity. A significance was found for falling starch in the tips of the tertiary roots by their behaviour after amputation of the primary root. At this juncture the secondaries take on the property of the primaries and the tertiaries behave diageotropically like secondaries. Without falling starch, it is held that this could not occur.

In 1904 Darwin was President of the Botanical Section of the British Association and gave an address which dealt very fully with all the recent work in support of the Statolith Theory. He dealt in addition especially with the problems of diageotropic organs and the phenomenon of rectipetality by which

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a freshly curved organ may recover its straight direction after the stimulus has ceased. An early contribution on diageotropic leaves (II, 1881) comes up for mention in this relation.

In 1906-7 Darwin gave a course of lectures in London University which were published in the "New Phytologist." There he expounded his satisfaction with the views of Semon who in his book "Die Mneme" (1904) provided a non-psychological nomenclature for dealing with vital behaviour of which the primary conception is the registration of stimuli as "engrams" in the organism. These become associated with one another by repetition and can be called forth by one component of an original association of stimuli. The association of heliotropic curvature with a light-stimulus would be an example of this.

He also stressed the quantitative studies of the relations between stimulus and response initiated by Fitting in 1905. Fitting dealt with the effects of intensity and duration of stimuli, with the result of weak intermittent stimuli, and with the reaction to opposed stimuli, of equal or different duration, in serial alternation. An early contribution by Darwin (V, 1888) dealt with the related problem of which angular position of a stem gives rise to maximal geotropic stimulus. In 1908 Darwin was President of the British Association and delivered an inaugural address to the whole Association. Addressing a general audience he presented the wider aspects of growth-curvatures as a problem of biology and adaptation. He drew parallels between the behaviour of plants and that of Protozoa as studied by Jennings in his work of 1904. These were both presented as acquired habits; and changes of structural morphology in relation to external stimuli, as studied by Goebel, were brought into the same view of Habit. He admitted in this address that he liked a blended mixture of psychology with physiology, and remarked that he generally got into hot water with the psychologists for this impropriety.

Some of his scientific contributions not yet noted dealt with the subject of rhythm in plants and this found mention under the discussion of Habit. The environment of the plant manifests a perpetual rhythm of day and night and the plant shows rhythmic habits in correspondence. Many of these rhythms of movement persist for a time when constant light or constant dark is substituted for the normal alternation. The "sleep movements" of leaves show this persistent rhythm. At several dates in association with Miss Pertz, who co-operated in much of his research work in the Cambridge Botany School, he tried the production of rhythms, consisting of up and down geotropic curvatures, by exposing plants to opposed geotropic stimuli for alternate periods of 30 minutes. This subject was dealt with in contributions (VI, 1892) and (VIII 1903). In this treatment he used the "intermittent" klinostat designed by his brother Horace for this special work.

His last contribution (XII, 1908) was a further critical study of the early

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subject of the geotropic curvature of seedling shoots fixed by their tips in a glass tube. This was sent as a contribution for the Festschrift to Professor Wiesner of Vienna. Professor Wiesner had as long ago as 1881 expressed his dissent from most of the conclusions about the significance of circumnutation set out in "The Movement of Plants," but he had combined his criticism with such courtesy and personal friendliness that the best human relations were always maintained. And so Darwin's last contribution to this subject was sent to honour his earliest critic.

The problems of biology endure for ever, and as an epilogue to the phase thus briefly pictured we should note the prologue to a new phase of the investigation of growth-movements which is now in full swing. It is based on the discovery of "growth-substances" which can easily be isolated from plant-tissues. These, applied locally, will accelerate the growth of one side of a shoot as against the other side with a smaller dose or none; thereby a typical growth-curvature results. These actual substances are continually secreted by growing points and one-sided light or gravity alters the amount of them and more especially determines that they drift down more to the side destined for greater growth. The connection between the "sensory region" and the "motor region" in plants is thus brought about by diffusing chemical hormones and not by any conduction of impulses analogous with nervous activity.

II.—Darwin's Researches on the Control of Water-Loss by the Living Plant.

The phenomenon of the control of loss of water by plants growing with their leaves in air which is subject to considerable variations of dryness, and intense variations of radiation has long interested philosophical biologists, who seek to make an orderly array of the complexity of physical, physiological and biological factors that are at work maintaining the survival of the individual plant.

This field of work had a great attraction for Darwin, coming second only to the study of plant movement and having many aspects in common with it, as a problem of adaptation. For the collection of data on water-loss he invented or developed a variety of special pieces of apparatus. With each new one he re-surveyed the fundamental problems several times between the years 1897 and 1916. To give a chronological account of the seven papers published in that period would, then, involve a good deal of repetition. To avoid this the present notice of his work is a synthetic one in which the time-sequence is ignored and the whole body of results is rearranged under headings of the different major factors that affect water-loss. As a preliminary to these divisions we present four sections, characterising the various sets of influences that have to be taken into account.

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The Physical Factors determining Water-Loss.—Any damp object losing water to the surrounding air by evaporation may be presented formally as a physical system which has certain significant attributes, determining the rate of water-loss. The parts of this system, entitled the source, the sink and the diffusion path, are conceived as being located at three different positions in space. We may characterise them as follows: 1. The source where water is vaporised and the concentration of water-vapour, or humidity, E, is highest, in any steady state of the system. In a leaf this is located at the surface of the mesophyll cells. 2. The sink where the vapour passes out of our field of enquiry. This is located somewhere in the outer air. Here the local humidity, e, will be least; the difference, E — e, being the prime determinant of rate of water-loss. In natural conditions e is an independent meteorological variable. Lying between these is (3) the diffusion-path of flow, constituted by the intercellular spaces and the stomatal pore. The variable attribute of this part of the system is the cross-section of the path, which we may express as a resistance to diffusion, K, rising as the cross-section is diminished locally.

Factors of General Physiology affecting Water-Loss.—On the physical properties of our system, which is losing water, there are superposed certain vital properties general to all cells whether they are specially concerned with water-loss or not. One of these properties is the local production of heat, owing to respiratory metabolism. This will accelerate vapour-production at the source and enable a supersaturation value of E to be maintained there, so that the cell can distil off water even with a saturated value of e at the sink. This local production of heat will cause water-loss to be in excess of physical expectation, also, at all lower values of e.

A second factor of general physiology arises from the increased permeability of protoplasm that is associated with light. This altered state is known to affect metabolism and may also increase the vaporisation possibilities.

Factors of Special Physiology: Guard-Cells.—These highly individualised cells in the epidermis of leaves have special functions in relation to water-loss. As they are placed like throttle-valves round the openings of the intercellular diffusion-paths, they directly affect the transverse section of this path, and so the diffusion-resistance, E. If they develop a large amount of osmotic substances they will withdraw water from the adjacent epidermis-cells and expand, thereby opening the stomatal pore. The guard-cells may thus be said to compete with the epidermis for the available water. With low osmotic contents the guard-cells may lose water to the epidermis and become so flaccid that the pore is closed. The guard-cells appear then to be capable of exercising an independent initiative in the control of water-loss. The manner of their working is to be explored later.

Factor of Biological Control: Stimulus to Guard-Cell Movement.—The most difficult problems for elucidation in this field of enquiry are those which arise

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when we consider the form-changes of the guard-cells as a special example of plant movement and then start to enquire precisely what changes in the environment act as stimuli to movement, and the nature of the chain of processes between the stimulus and the reaction. This reaction only takes place with living protoplasm, so the general state or "tonus" of protoplasm must affect the nature of the reaction.

Such are the four strata of causal relations that interact on one another to determine the rate of water-loss of a leaf under the natural range of variation of the humidity and illumination of the leaf's environment. Though all the data could, in theory, be gathered merely by weighings, yet the interpretation of the variations of rate of loss that would be recorded involves a large part of the physiology of plants.

Darwin worked through this field of phenomena at different times with different instruments, the principles of which we shall next describe and then pass to the results that he obtained.

Darwin's Instruments for the Investigation of Water-Loss.

Darwin had a striking natural talent for designing simple pieces of apparatus in response to his needs as an investigator. A chronological series of those employed in his work on water-loss is set out below. The potometer, the hygro-scope and the porometer are so simple that they consist essentially of a principle clothed in the minimum of glass, rubber and cork. The electric recorder was, of course, a highly complex scientific instrument, but this was designed by his brother, Horace Darwin, primarily for non-biological purposes.

The Potometer (1884).—This consisted of a glass reservoir of water into which a cut shoot was fixed air-tight so that it sucked out the water and created a reduced pressure in the apparatus. Through a side capillary tube air bubbles are drawn in succession from outside to neutralise the reduced pressure. The rate of the inflow of the air bubbles gives a measure of the rate of uptake of water by the shoot. When steady conditions have once been established in the cut shoot, this "potometer-rate" gives an accurate measure of the loss of water by the surface of the shoot to the surrounding air. Similar, but less sensitive, forms of this apparatus had been published by other physiologists.

The Horn-Hygroscope (1897).—This little instrument is constructed out of small strips of horn-shaving specially treated so that the strip expands on the face exposed to moisture: it thus curls away from the surface of a leaf to a degree determined by the rate at which water vapour is pouring out from that surface. The strip is provided with an index bristle and a tiny cardboard scale and can be transferred from one leaf to another to give an empirical but quantitative measure of the water-loss, at any spot on the stomatal surface.

[page] Francis Darwin. xv

The Electric Recorder of Leaf-cooling (1904).—An evaporating leaf can be arranged so that, by contact with a platinum wire grid, it absorbs heat from this grid. If the wire is part of the circuit of an electric-resistance-thermometer, the rise of the wire's resistance is a measure of its cooling and so a very sensitive recorder of the rate of evaporation from a leaf, minute by minute, can be constructed.

The Porometer (1911).—This instrument was a new invention by Francis Darwin and differs from all the others in that it gives a measure of the degree of openness of the stomatal pore. It provided a new power of analysis, as the change of total water-loss, which the other instruments alone record, can be examined as to whether it is associated with change of the aperture of the diffusion path (R) or is due only to alteration of the humidity of the source and the sink. The porometer in its simple form consists of a small glass bell-chamber which is sealed to the stomatal surface of a leaf by gentle pressure on a gelatine disc. By sucking on a branched side-tube negative pressure can be established in the bell-chamber and a water column raised from an open water vessel. The rate at which, when the suction is stopped, the sinking water column draws air through the stomata and intercellular spaces of the leaf gives a primary "rate of air flow" value which measures the grade of openness of the pores. The square root of this value gives a measure of the diffusive capacity of the pore for the outward drift of water-vapour.

The value of a measure of the rate of flow of air through stomata to give evidence of their state of openness was recognised by N. J. C. Muller in 1873, but his procedure was too complicated for much scientific application.

The Elimination of Stomatal Control (1914).—Darwin thought out a technique by which water-loss could be studied in a leaf from which the factor of control by guard-cells had been eliminated. The preparation of such a leaf involves rather drastic treatment, but shoots of hardy evergreens seem to submit to this without serious injury. The surfaces of the stem and leaves are rubbed over with vaseline so that all the stomatal apertures are blocked. After this a definite number of razor cuts is made through the blade of each leaf so as to allow the escape of water vapour from the intercellular spaces to the outside sink by cut openings which are not throttled by guard-cells. The number of cuts made is such as to give approximately the normal water-loss per leaf. This water-loss is sometimes directly measured by weighing, but usually by attaching the shoot to a potometer for measuring water-intake as a guide to water-loss.

The Effect of Dryness and Humidity on the Water-Loss of Plants.

Having now formalised the transpiring leaf as a physical system consisting of source plus diffusion path plus sink, and described the instruments with which the rate of water-loss has been investigated we pass to consider the actual

[page] xvi Obituary Notices.

behaviour of the living plant and the problem of how it comes to depart from that of a non-living system.

In the present section we will deal with Darwin's work on the effects of changes in humidity, leaving the experimental work on illumination to the next section. For the integrated water-loss from a simple physical system we should start with the expectations that (1) decrease of humidity at the source would decrease water-loss, while (2) decrease of humidity at the sink would increase loss.

In experimental work Darwin applied the first by cutting through the water channels in the leaf-stalk: and the second by moving the leaf still on the plant, from damp air to dry air.

Reduction of Humidity at the Source.—The effect on the water-loss of cutting or clamping the leaf-stalk can be observed with the horn hygroscope placed on the stomatal surface of the leaf, or recorded by placing the non-stomatal surface in close contact with the wire grid of the electric recorder. Directly after cutting, either method shows a quite unexpected increase of water-loss which may get greater and greater for some 10 minutes, after which the water loss starts to decline and thenceforward goes on making a steady approach towards zero. There can be no physical explanation of the temporary rise of water-loss. With the system in question, this could only be interpreted as due to wider opening of the stomatal pore. By the direct use of the porometer the opening was established, even to the extent that the diffusive potentiality of the stomata might be raised 1.8 times, immediately after cutting, and that this increase might last for 25 minutes before decrease sets in. To what was this strange opening of the pore to be attributed? Cutting of leaf-stalks is no part of normal biology, but a clue to the nature of the mechanism involved might be revealed here. Clearly any loss of water by the guard-cells should close them, but we observe an opening. Could it be due to deturgesence of the surrounding epidermis lowering the pressure with which it opposes the opening of individual guard-cells?

This transitional increase of water-loss soon stops and there follows a rapid decline of water-loss, which is obvious and well-marked with all instruments. It might have been possible that the decline was wholly due to falling humidity at the source, but Darwin's application of the porometer to withering leaves proved that the stomata shut progressively and rapidly so that the rate of water-loss is greatly reduced and thereby drying up is postponed. What then causes the guard-cells to close in this second phase though they opened in the first phase? By this time, the general water-loss of the leaf has proceeded further as its supplies are completely cut off; and the guard-cells themselves may be held to undergo loss of turgidity and mechanical collapse. Or possibly, as Darwin suggests, a stimulus may be transmitted to the guard-cells arising out of the general deturgescence that has taken place in the leaf.

[page] Francis Darwin. xvii

Variation of Humidity at the Sink.—We now pass from the unnatural condition examined in the previous section to consider the effect of alteration of the relative humidity of the surrounding air on the rate of the plant's water-loss. This may be said to be the most irregular natural variable in the biological experiences of plant life.

The physical expectation of the effect of reduced humidity at the external sink is, of course, increase of water-loss by reduction of e values: and thus the opposite of reduction of E, at the source. Within certain limits this expectation may be fulfilled and a steady increased water-loss be maintained in a lesser humidity provided this lesser humidity is itself still very high. In the region of experimentation that Darwin explored the variations of humidity used are more drastic and the more immediate effects of change are alone investigated. He found it was very rare to get a plant which did not react in the reverse of physical expectation when brought from a greenhouse into a dry laboratory, and thereby exhibit a reduction of water-loss. This reduction we should attribute to biological regulation by the reaction of the guard-cells. As we have indicated higher up, the behaviour when the humidity at the sink is lowered partly depends on the productive power of the source. Aloi had pointed out, and Darwin confirmed, that if the soil of a pot plant is abnormally saturated with water then a higher rate of water-loss initiated by a drier sink may be maintained without any regulatory closure of stomata. Here we should hold that no deturgescence took place.

In attempting to distinguish the part that is played by stomata, it is very important that we should acquire knowledge of the behaviour of the source when the superposed control of guard-cells is removed. This piece of analysis Darwin succeeded in carrying out by the waxing and cutting technique which has been already described (see p. xv). Treating leaves in this way, he arrived at the water-loss of the shoots by measuring their intake from a potometer. These experiments were carried out for a wide range of relative humidities of the external air, from 60 per cent, to 95 per cent.

Here in the absence of all guard-cell control the physical expectation is completely realised and the water-loss falls off in a straight line relation to the rising humidity at the sink. An array of data of this type had not been brought forward before and when set out graphically it was obvious that water-loss was not drifting towards a zero value in air of 100 per cent, relative humidity but that the slope indicated a quite considerable water-loss in saturated air, falling to zero only in a supersaturated state, located on the graph at about 105 per cent, relative humidity. Here than was a demonstration of the reality of the physiological expectation described on p. xiii that the continuous heat-production in respiration must give a "distillation effect."

Having acquired a picture of the behaviour of water-loss in the absence of guard-cells we may return to the normal leaf with its biological controls.

[page] xviii Obituary Notices.

With normal plants in soil of normal humidity nearly all the observations that Darwin made with dry air indicate a special initial phase of increased water-loss followed by an adjusted phase in which the water-loss becomes markedly decreased. The electrical recorder of the heat-absorption that is a concomitant of increased evaporation gives the clearest records of this initial effect. Such an increase is in accord with physical expectation on reduction of e. Whether there is an actual wider opening of stomata associated with it was not tested by the porometer, as the humidity of the air inside the porometer would require special controlling complications. This initial phase may last perhaps 20 minutes. After 30 minutes or so there sets in a marked reduction of water-loss when the potted plant is brought from the greenhouse to the drier laboratory. Numerous measurements with the horn-hygroscope prove the generality of this effect. The reduction comes on before the eye detects any appearance of wilting in the leaf-blade. It can only be attributed to closing of the stomatal aperture by deturgescence of the guard-cells. The stimulus for the production of this biological control of water-loss Darwin would attribute to an initial general deturgescence of the leaf.

We note that the initiation of this stomatal closure is less humidity at the sink, and yet it closely resembles the closure initiated by less humidity at the source. As physical causes, these two lead to opposed effects upon rate of water-loss. When we seek some common aspect to suggest as the directing cause of change of guard-cells, we find it in the state of reduced water content of the leaf tissues intervening between the leaf water-supply and the external air. The analogue of this on our physical system with water vapour drifting from E to e, the rate of flow being determined by E — e, is the fall of the mid-value —E + e /2— that can be produced indifferently by lowering either E or e alone.

It is in this general deturgescence rather than in direct deturgescence of guard-cells that Darwin sees the stimulus which brings about the reaction of the guard-cells. Since the date of Darwin's experimentation it has been shown by Thoday that leaves shrink a lot in area under conditions of strong water-loss such as exposure to direct sun. This shrinkage is easily measured in a linear direction between Indian ink marks on the leaf surface and it might be that a series of such measurement would give the best direct index of the degree of openness of stomata.

The Effect of Illumination and Darkness upon the Water-Loss of Plants.

Taking up the factor of illumination we leave simple physical expectations behind us, as light has no primary effect upon the rate of evaporation from damp surfaces to air. We have seen, however, in our introduction that light

[page] Francis Darwin. xix

might increase the efficiency of the sources of water-vapour (see p. xiii). We have now to bring together the experiments made by Darwin which seek out evidence of this general effect of light as well as such special effects of light on guard-cells as can be shown to take part in the biological control of water-loss. As a foundation for this special enquiry we have the general view of the pioneer workers that the stomata on leaves are open by day and close at night.

The first experimental finding that we have to record is the full confirmation of the reduction of water-loss that follows artificial darkening of leaves during the day-time. Numerous observations with the potometer and the horn-hygroscope and a few with the electric recorder show that this effect is widely characteristic of typical leaves. But this is an integrated effect and calls for analysis. The special waxing and cutting technique introduced by Darwin allowed him to examine the behaviour of the evaporating source apart from guard-cell control. The measurements of intake by these shoots drawing water from a potometer showed higher values in the diffuse light of a north window than in darkness. The ratios for the two states are very variable, ranging from no change to two-fold water-loss. Susceptibility seems to be seasonal as the summer increase is much greater. The average ratio for a very large number of cases is 132/100. From these data one seems justified in holding that light-effects, of the nature of permeability changes, increase the evaporating efficiency of the mesophyll cells.

By applying the porometer as a measure of factor II it was made quite clear that a considerable increase in the pore-opening is associated with light. As light increases both of the factors making for increased water-loss, Darwin attempted the rather difficult task of distributing the total effect between the two factors. The nature of the survey was to study how closely the variations of diffusion-potentiality, derived from the porometer rate of flow, follow the drift of total water traffic measured by the potometer. The agreement is not very detailed, so that we cannot yet be sure of the correctness of the view that the influence of light on the guard-cells is by far the more important factor.

It is interesting at this stage to point out a marked difference in the process of closing of stomata when brought about by darkening instead of by drought. With darkening there is no sign whatever of the special transitional phase observed in drought in which, for some minutes, the stomata become more open. With darkening there is a slow steady decline of water-loss from the moment of darkening. This is well seen in a few records with the electric recorder. We conclude that light has only a single type of effect, a physiological acceleration of water-loss, while drought has two types of effects, physical in increasing loss and physiological in reducing loss.

A more recent discovery in the physiology of guard-cells made since Darwin's work cannot be passed over without reference here. Iljin stressed the importance of the fact that the starch, which is so characteristic a content of guard-

[page] xx Obituary Notices.

cells, readily passes over into sugar by a simple exposure to light thereby generating osmotic substance which will raise the water absorption and the turgor. On darkening the process is reversed. The light and darkness are held to work through the catalysts concerned in the starch-sugar conversion. Humidity of air has similar relations: exposure to dry air leads to starch-formation while humid air re-forms the osmotic sugar content. Apparently we can now feel safe in interpolating this protoplasmic control of the amount of osmotic substance into the chain between the initiating dry air and the final change of form of the guard-cells, but the problem of how the initiatory process comes to affect the metabolic machinery of the guard-cells is still left on our hands.

Nocturnal Closure of Stomata: Day and Night Rhythm.— The approach that was made by early workers to the influence of light upon stomata was to stress a phenomenon called the Nocturnal Closure of Stomata and to enquire about its mechanism, the range of its occurrence and its biological advantages. Darwin surveyed this field with much thoroughness by the aid of his delicate indicator of relative water-loss—the horn-hygroscope—and the range of plants that the Cambridge Botanic Garden could provide. Subsequent work with the porometer showed that it is really closure of stomata that reduces the water-loss at night. It appears that closure begins well before sunset and reaches its maximum about 10 p.m. In the morning reopening begins before sunrise. Darwin was much interested to enquire whether the constant repetitions of this rhythm of change had led to any inherent periodicity in the sequence of stomatal states. As a test for this he used the formal method of finding out whether artificial light would bring on opening when the stomata are in normal nocturnal closure. He found that the stomata are more readily induced to open by light applied at the end of the closure in early morning than by light applied during the closing period in the early evening. The responsiveness of the cells is different at these two periods of time and thus evidence is provided of some inherent periodicity in the protoplasmic states. Darwin concluded that there is a definite inherent reaction of nocturnal closure and this has been more clearly brought out in experiments since his date. Maskell has observed that powerful constant artificial light all through the 24 hours will not prevent the stomata of a leaf closing during the early evening to a minimal opening at 10 p.m. It is interesting to find that in this constant strong light they do not remain shut, but soon start to reopen and may be fully open again long before the normal time.

Darwin speculated not very successfully about the biological advantage of nocturnal closure, although he found that aquatic and marsh plants hardly show it and it is only slight in leaves which have a special "sleep" position of their leaves at night.

[page] Francis Darwin. xxi

III.—Darwin's other Biological Works.

Omitting any reference to the contents of the large number of short papers that Darwin published, there are certain of his scientific activities that led to the production of books and so call for mention here.

His well-known class-book "The Practical Physiology of Plants" appeared in 1894. This book was built up on his experience in holding a practical class in this subject for many years. No book of this type had been available in English before, as Sachs' "Experimentalphysiologie" of 1865 had never been translated. Darwin's book ran into several editions.

He also published a small book entitled "Elements of Botany" (1895) which presented the lectures that he had given in Elementary Biology to medical-students.

The "Makers of British Botany" (1913) was a volume of composite authorship to which Darwin contributed an article on Stephen Hales who, though not a botanist, created, all by himself in the exercise of his power as an investigator, many philosophical aspects of plant physiology.

Darwin's edition (1903) of "A Naturalist's Calendar" was referred to in the biographical part of this notice.

To end this list we may mention that Darwin lectured for a number of years on the Natural History of Plants. Finally he printed his lecture-notes with descriptions of all the plants to be examined in the practical work. Sets of sheets amounting to 100 folio pages were bound up for private circulation, but never published.

F. F. B.

Harrison and Sons, Ltd., Printers, St. Martin's Lane, London, W.C.2.