"On The Protection of Leaves from Water"
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Charles Darwin's unpublished last book
An introduction by Christine Chua & John van Wyhe
There are three volumes in Darwin's manuscripts and papers from the Darwin Archive at Cambridge University Library with notes from his research on "bloom on leaves and fruit" (CUL-DAR66-68). This was the book Darwin was working on when he died aged 73. His son Francis explained: "His researches into the meaning of the 'bloom,' or waxy coating found on many leaves, was one of those inquiries which remained unfinished at the time of his death. He amassed a quantity of notes on the subject". LL3: 339.
Indeed there is a large quantity of notes that survive, 308 have been catalogued. The Darwin Online project has transcribed 284 of these for the first time. The OED defines bloom as "The delicate powdery deposit on fruits like the grape, plum, etc., when fresh-gathered, and on certain plant-leaves." Today this waxy covering is called the cuticle.
Like almost all of Darwin's research projects, including his theory of evolution itself, the research went on behind the scenes for decades while earlier work was still in the foreground. The first dated note is from 31 March 1863 and the last is 7 April 1882. Darwin died twelve days later on 19 April. The book was never published by Darwin nor his son Francis, who was his assistant and intended co-author in the observations and experiments in this study. In 1886 Francis did publish a small part of the research that he had undertaken: On the relation between the 'bloom' on leaves and the distribution of the stomata. Journal of the Linnean Society of London, (Botany) 22: 99-116. (F1805)
Darwin drafted four title suggestions for the unpublished book (CUL-DAR68.41):
The Protection of Glaucous Leaves
The Protection of Leaves by Bloom by Charles Darwin
The uses of the waxy Secretion or Bloom on Plants.
by Ch. & Francis Darwin
and in CUL-DAR66.42-43:
On "The Protection of Leaves from Water by Ch Darwin
There are two introductory drafts, one in CUL-DAR66.44v (19 April 1881) and another in CUL-DAR66.42-43 (1875?), where Darwin states that "The first point to ascertain was whether the loss removal of the bloom or Pruina (?) — or waxy secretion wd injure leaves in any fruit or leaves in any degree." He explains he was "much struck with…sensitive Plants as Drosera & Dionæa not affected by stream of water" and later came to the conclusion that "if repelling water was one of the uses or effects of bloom, there were probably many other[s] & whole subject seem[s] worth investigating."
The methods of testing were typically wide ranging include dripping, sponging, spraying, syringing, squirting, wetting, dipping, waving in liquids of varying temperatures, dusting with chalk or pumice and shaking the fruit and leaves to determine the degree of absorption, adherence and repellence and the degree of damage the action might cause. Various stimulants were introduced to the specimens and the results closely tabulated, with assistance from Leonard and Horace Darwin in addition to Francis. The age of the specimens, the places where they were exposed were also considered. This was typical Darwin thoroughness.
Darwin wanted to know if 'bloom' was a product of natural selection as it conferred some advantages to the plants either at the present time or had done in their evolutionary past. He thought it straightforward that bloom was "adapted to prevent water lodging on the leaves" (CUL-DAR66.42-43) and that "I soon came to conclusion that if repelling water was one of the uses or effects of bloom, there were probably many other & whole subject seem[ed] worth investigating." And his experiments had indeed suggested many other adaptations: "We shall see many uses — It seemed to me to protect from insects — from evaporation water-plants─ From water — Salt-water — farm Plain Water, from excess of light — colour of leaves changed by removal of bloom (Parasitic fungi)". (CUL-DAR66.44v)
Browsing through Darwin's notes reveals more than his experimental records, they often record snapshots of his daily life. For example, Darwin would walk out to his hothouse and examine or experiment with some plants. He would then go for his walk around the sandwalk. Afterwards he would stop by the hothouse again to see if there were any changes to his plants and then he would return to his study and jot down the notes now transcribed here.
See all of Darwin's notes on 'bloom' below: