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