The facts now given on dimorphic and trimorphic plants are
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important, | important, 1869 1872 | | importance, 1866 |
| first, 1869 1872 | | firstly, 1866 |
| may conclude 1869 1872 |
| are thus led to infer, as previously remarked, 1866 |
| bond which 1869 1872 |
| law or bond 1866 |
| connects 1869 1872 | | connecting 1866 |
| with that 1869 1872 |
| and of first crosses, with the infertility 1866 |
| offspring, and we are led to extend the same view to first crosses and hybrids; 1869 1872 |
| and hybrid offspring; 1866 |
| importance, 1869 1872 | | portance, 1866 |
| respect 1872 | | respect, 1866 1869 |
| whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when united 1872 |
| except 1866 1869 |
| certain ways. 1872 |
| certain characters in their reproductive organs,—such as in the relative lengths of the stamens and pistils, in the size, form, and colour of the pollen-grains, in the structure of the stigma, and in the number and size of the seeds. 1866 |
| their reproductive organs, and yet be sterile when united in certain ways. 1869 |
| 4 blocks not present in 1859 1860 1861 1866 1872; present in 1869 | | With, dimorphic plants, the unions between the two distinct forms are alone quite fertile, and produce quite fertile offspring, whilst unions between individuals belonging to the same form are more or less sterile; so that the result is exactly the reverse of what occurs with distinct species.
With dimorphic plants the resultant sterility is quite independent of any difference in general structure or constitution, for it arises from the union of individuals belonging not only to the same species, but to the same form.
It must, therefore, depend on the nature of the sexual elements, which are so adapted to each other, that the male and female elements occurring in the same form do not suit each other, whilst those occurring in the two distinct forms are mutually suited to each other.
From these considerations, it seems probable that the sterility of distinct species when crossed, and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any general difference in structure or constitution.
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| 2 blocks not present in 1859 1860 1861 1869 1872; present in 1866 | | With these differences and with no others, either in organisation or constitution, between the several forms, which are all hermaphrodites, we find that their illegitimate unions and their illegitimate progeny are more or less sterile, and closely resemble in a whole series of relations the first unions and the hybrid offspring of distinct species.
We are thus led to infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to similar differences confined to their reproductive systems.
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| also 1872 | | indeed 1866 1869 |
| crosses, 1869 1872 | | crosses 1866 |
| OMIT 1869 1872 |
| between the same two species, 1866 |
| species cannot 1869 1872 | | cannot 1866 |
| a second 1869 1872 | | the other 1866 |
| facility. 1872 | | facility; 1866 1869 |
| ..... 1872 | | difference 1866 1869 |
| ..... 1872 | | facility 1866 1869 |
| ..... 1872 | | making 1866 1869 |
| ..... 1872 | | reciprocal 1866 1869 |
| ..... 1872 | | crosses 1866 1869 |
| ..... 1872 | | fertility 1866 1869 |
| ..... 1872 | | their 1866 1869 |
| ..... 1872 | | offspring 1866 1869 |
| ..... 1872 | | attributed 1866 1869 |
| ..... 1872 | | either 1866 1869 |
| ..... 1872 | | female 1866 1869 |
| ..... 1872 | | element 1866 1869 |
| ..... 1872 | | one 1866 | | first 1869 |
| ..... 1872 | | species 1866 1869 |
| ..... 1872 | | having 1866 1869 |
| ..... 1872 | | differentiated, 1866 1869 |
| ..... 1872 | | reference 1866 1869 |
| ..... 1872 | | other 1866 | OMIT 1869 |
| ..... 1872 | | sexual 1866 1869 |
| ..... 1872 | | element, 1866 | | elements 1869 |
| OMIT 1872 |
| in 1866 |
| of the second species in 1869 |
| ..... 1872 | | higher 1866 1869 |
| ..... 1872 | | degree 1866 1869 |
| ..... 1872 | | second species. 1866 | | converse case. 1869 |
| concluded 1872 |
| came on general grounds to the same conclusion, namely, 1866 |
| came to this same conclusion, namely, 1869 |
| 10 blocks not present in 1859 1860 1861 1869 1872; present in 1866 | | Finally, we are naturally led to inquire for what useful end have plants been rendered reciprocally dimorphic and trimorphic?
A wide-spread analogy clearly gives us the answer as far as the immediate cause is concerned, namely, to prevent the pollen of each flower acting on the stigma of that flower.
We see this effected in a host of flowers by the most curious mechanical contrivances, as I have shown with Orchids, and as could be shown with many plants of many other orders.
There are also numerous plants, called dicho-gamous by C. K. Sprengel, in which the pollen and stigma are never mature at the same time, so that these plants can never fertilise themselves.
There are many flowers, which, though they have their stigmas and pollen mature together, and which do not present any obstacle to self-impregnation, yet nevertheless are almost always fertilised by surrounding varieties when growing in the vicinity, as shown by the character of their seedlings.
Then, again, we have many flowers with separated sexes borne on distinct plants, or on the same, which inevitably prevents self-fertilisation.
Lastly, in accordance with the great principle prevailing throughout nature, of the same end being gained by the most diversified means, we find in dimorphic and trimorphic plants, in which self-fertilisation is not checked by any of the above-specified means, that this has been effected by the pollen of each flower, and consequently of all the flowers of the same form, having been rendered more or less impotent on their own stigmas; so that its action is easily and wholly obliterated by pollen habitually brought by insects from other individuals and forms of the same species.
In searching for the cause of dimorphism and trimorphism in plants, we may, in my opinion, safely go one step further, and conclude that the pollen has been prevented acting on the stigma of the same flower, in order to give vigour to the offspring by leading to the union of two distinct individuals.
But on this view it is not a little remarkable that the end has been gained, in the case of dimorphic and trimorphic plants, at the expense of all the plants of the same form being rendered more or less sterile when united, and producing more or less sterile offspring.
With respect to the steps by which it is probable that plants have been rendered dimorphic and trimorphic, want of space prevents my entering on the subject; but I will add that there is no special difficulty in this having been effected through variability, through the good gained by the prepotency of one sort of pollen over another, and through the accumulative action of natural selection.
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