CHAPTER XXIV.

BOTANICAL WORKS (1862–86).

Darwin’s botanical works are referred to separately, and receive more systematic treatment than the others, in the great “Life and Letters.” They form, together with the botanical letters, the subject of the seventh to the twelfth chapters in the last volume. It will therefore be unnecessary to treat them in any detail, although they form some of the most important and interesting of all his biological investigations.

_Fertilisation of flowers._--“The Fertilisation of Orchids” was the first published of the botanical works, appearing in 1862, followed by a second and greatly altered edition in 1877. The object of the work “is to show that the contrivances by which orchids are fertilised are as varied and almost as perfect as any of the most beautiful adaptations in the animal kingdom; and secondly, to show that these contrivances have for their main object the fertilisation of the flowers with pollen brought by insects from a distant plant.” Even in 1837 Darwin had written in his note-book, “Do not plants which have male and female organs together [_i.e._ in the same flower] yet receive influence from other plants? Does not Lyell give some argument about varieties being difficult to keep [true] on account of pollen from other plants? Because this may be applied to show all plants do receive intermixture.” (Quoted in the “Life and Letters.”) In 1841, Robert Brown, the distinguished botanist, advised Darwin to read Sprengel’s “Secret of Nature Displayed” (Berlin, 1793). The result was to encourage and assist Darwin in his work on fertilisation of flowers by insects, and to bring about the first due recognition of Sprengel’s merits, long after his death.

“_The Effects of Cross- and Self-fertilisation in the Vegetable Kingdom._”--This work has a very direct bearing on that last mentioned. Darwin speaks in the Autobiography “of having come [in 1839] to the conclusion in my speculations on the origin of species, that crossing played an important part in keeping specific forms constant.” Later on he came to see that the advantage of crossing is more direct, and results from the greater vigour of the offspring over those of self-fertilised plants. The object of this work, published in 1876, was to prove this point by experimental evidence of sufficient amount, and to show in numerous cases, by measurements of height or weight, or by counting the number of seeds produced, that cross-fertilisation invariably tends towards the greater vigour of offspring.

Hence the motive cause for the marvellous adaptations by which cross-fertilisation is ensured was supplied.

[Sidenote: FORMS OF FLOWERS.]

“_Different Forms of Flowers on Plants of the Same Species_” was published in 1877, and a second edition in 1880. This work, like so many others, had been largely anticipated by the author’s original papers to scientific societies, in this case to the Linnean. The papers were combined, brought up to date, and with the addition of much new matter constituted the volume. The chief part of the work is concerned with heterostyled plants, viz. species which bear different kinds of flowers chiefly distinguished by the lengths of the pistil and stamens. As many as three different forms occur in Lythrum. In this work it is shown that each of the forms, although possessing both kinds of sexual organs, is adapted to be fertilised by the pollen of another form, and that such offspring are more vigorous than those produced by fertilisation by the same form. He furthermore showed that the offspring of “illegitimate” parentage (viz. those which were fertilised by the same form) possessed, in certain respects, a close resemblance to hybrids among animals. He remarks in his Autobiography, “No little discovery of mine ever gave me so much pleasure as the making out the meaning of heterostyled flowers.”

In addition to the heterostyled flowers, the other differing forms borne by the same plants are considered, including the cleistogamic species, in which minute closed flowers are borne as well as the ordinary open ones. The former are wanting in the scents and colours of ordinary flowers, and are specially adapted for self-fertilisation, and the production of “an abundant supply of seeds with little expenditure.”

“_Climbing Plants._”--The subject of this volume was published as a paper before the Linnean Society in 1864. After being corrected, the material was brought out as a volume in 1875. Darwin, as he tells us in the Autobiography, was first led to study the subject by a paper by Asa Gray, which appeared in 1858 (Proc. Amer. Acad. of Arts and Sciences). Writing to Asa Gray, August 4th, 1863, he said, “My present hobby-horse I owe to you, viz. the tendrils.” One of the most interesting results brought forward in this work is the fact that the upper growing part of a twining stem bends to one side and then travels slowly round, between two and three hours being required for each revolution, in the case of the hop growing in a room and observed at the period of most active movement. The circle swept at the 27th revolution was 19 inches in diameter. In the case of this plant the three youngest internodes (or joints), and never less than two of them, were concerned in the movement; “by the time the lower one ceased to revolve, the one above was in full action, with a terminal internode just commencing to move.” The object of this movement is to strike some object round which the plant may twine. A much grander example was seen in _Ceropegia Gardnerii_, in which three long internodes and two short ones swept a circle over 5 feet in diameter, “at the rate of 32 or 33 inches per hour, in search of some object round which to twine.” The stem of the plant is not in the least twisted by this movement. Nearly all of the great divisions of twining plants, leaf-climbers, and tendril-bearers “have the same remarkable power of spontaneously revolving.”

[Sidenote: MOVEMENT IN PLANTS.]

“_The Power of Movements in Plants_” was published on November 6th, 1880. It embodies a vast amount of work carried on in conjunction with Francis Darwin. This volume bears a very direct relation to that last mentioned, as Darwin has explained in his Autobiography:--

“In accordance with the principle of evolution it was impossible to account for climbing plants having been developed in so many widely different groups unless all kinds of plants possess some slight power of movement of an analogous kind. This I proved to be the case; and I was further led to a rather wide generalisation, viz. that the great and important classes of movements, excited by light, the attraction of gravity, &c., are all modified forms of the fundamental movement of circumnutation.”

An extreme example of circumnutation has already been described in the revolving movements of the youngest parts of the stem of a twining plant.

The work evoked very great interest in this country, but was severely criticised by certain German botanists. The immense number of new observations must always have a very high value, whatever be the fate of the general conclusions, concerning which it may be remarked that Darwin’s conclusions have often been criticised before, but time has shown that he was right.

“_Insectivorous Plants_” was published July 2nd, 1875, but I consider it last, as the subject stands somewhat apart from the rest of his botanical works. The subject was suggested to him by noticing the insects caught by the leaves of the Sun-dew (_Drosera_) near Hartfield. He then studied in great detail the causes of the movement, and the sensitiveness of the gland-tipped hairs, finding that a piece of hair weighing 1/78000 of a grain causes one of them to curve inwards, and alters “the condition of the contents of every cell in the foot-stalk of the gland.”

The greater part of the work deals with the experiments on _Drosera_, which were extremely numerous and detailed. The remainder treats of other insectivorous plants, such as Dionæa, Pinguicula, Utricularia, etc. The methods of capture, the movements of the plants under the stimulus supplied by the living insect (or other animal), and the resulting changes in the plant-cells were not the only points studied. He also investigated the digestive secretion and its action upon the food absorbed by the leaves.