Haberlandt and Nemec ("Ber. d. Deutschen bot. Gesellschaft", XVIII. 1900. See F. Darwin, Presidential Address to Section K, British Association, 1904.) published independently and simultaneously a theory of the mechanism by which plants are orientated in relation to gravitation. And here again we find an arrangement identical in principle with that by which certain animals recognise the vertical, namely the pressure of free particles on the irritable wall of a cavity. In the higher plants, Nemec and Haberlandt believe that special loose and freely movable starch-grains play the part of the otoliths or statoliths of the crustacea, while the protoplasm lining the cells in which they are contained corresponds to the sensitive membrane lining the otocyst of the animal. What is of special interest in our present connection is that according to this ingenious theory (The original conception was due to Noll ("Heterogene Induction", Leipzig, 1892), but his view differed in essential points from those here given.) the sense of verticality in a plant is a form of contact-irritability. The vertical position is distinguished from the horizontal by the fact that, in the latter case, the loose starch-grains rest on the lateral walls of the cells instead of on the terminal walls as occurs in the normal upright position. It should be added that the statolith theory is still sub judice; personally I cannot doubt that it is in the main a satisfactory explanation of the facts.
With regard to the RAPIDITY of the reaction of tendrils, Darwin records ("Climbing Plants", page 155. Others have observed movement after about 6".) that a Passion-Flower tendril moved distinctly within 25 seconds of stimulation. It was this fact, more than any other, that made him doubt the current explanation, viz. that the movement is due to unequal growth on the two sides of the tendril. The interesting work of Fitting (Pringsheim's "Jahrb." XXXVIII. 1903, page 545.) has shown, however, that the primary cause is not (as Darwin supposed) contraction on the concave, but an astonishingly rapid increase in growth-rate on the convex side.
On the last page of "Climbing Plants" Darwin wrote: "It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them."
He gradually came to realise the vividness and variety of vegetable life, and that a plant like an animal has capacities of behaving in different ways under different circumstances, in a manner that may be compared to the instinctive movements of animals. This point of view is expressed in well- known passages in the "Power of Movement". ("The Power of Movement in Plants", 1880, pages 571-3.) "It is impossible not to be struck with the resemblance between the...movements of plants and many of the actions performed unconsciously by the lower animals." And again, "It is hardly an exaggeration to say that the tip of the radicle...having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements."
The conception of a region of perception distinct from a region of movement is perhaps the most fruitful outcome of his work on the movements of plants. But many years before its publication, viz. in 1861, he had made out the wonderful fact that in the Orchid Catasetum ("Life and Letters", III. page 268.) the projecting organs or antennae are sensitive to a touch, and transmit an influence "for more than one inch INSTANTANEOUSLY," which leads to the explosion or violent ejection of the pollinia. And as we have already seen a similar transmission of a stimulus was discovered by him in Sundew in 1860, so that in 1862 he could write to Hooker ("Life and Letters", III. page 321.): "I cannot avoid the conclusion, that Drosera possesses matter at least in some degree analogous in constitution and function to nervous matter." I propose in what follows to give some account of the observations on the transmission of stimuli given in the "Power of Movement". It is impossible within the space at my command to give anything like a complete account of the matter, and I must necessarily omit all mention of much interesting work. One well-known experiment consisted in putting opaque caps on the tips of seedling grasses (e.g. oat and canary-grass) and then exposing them to light from one side. The difference, in the amount of curvature towards the light, between the blinded and unblinded specimens, was so great that it was concluded that the light-sensitiveness resided exclusively in the tip. The experiment undoubtedly proves that the sensitiveness is much greater in the tip than elsewhere, and that there is a transmission of stimulus from the tip to the region of curvature. But Rothert (Rothert, Cohn's "Beitrage", VII. 1894.) has conclusively proved that the basal part where the curvature occurs is also DIRECTLY sensitive to light. He has shown, however, that in other grasses (Setaria, Panicum) the cotyledon is the only part which is sensitive, while the hypocotyl, where the movement occurs, is not directly sensitive.
It was however the question of the localisation of the gravitational sense in the tip of the seedling root or radicle that aroused most attention, and it was on this question that a controversy arose which has continued to the present day.
The experiment on which Darwin's conclusion was based consisted simply in cutting off the tip, and then comparing the behaviour of roots so treated with that of normal specimens. An uninjured root when placed horizontally regains the vertical by means of a sharp downward curve; not so a decapitated root which continues to grow more or less horizontally. It was argued that this depends on the loss of an organ specialised for the perception of gravity, and residing in the tip of the root; and the experiment (together with certain important variants) was claimed as evidence of the existence of such an organ.
It was at once objected that the amputation of the tip might check curvature by interfering with longitudinal growth, on the distribution of which curvature depends. This objection was met by showing that an injury, e.g. splitting the root longitudinally (See F. Darwin, "Linnean Soc. Journal (Bot)." XIX. 1882, page 218.), which does not remove the tip, but seriously checks growth, does not prevent geotropism. This was of some interest in another and more general way, in showing that curvature and longitudinal growth must be placed in different categories as regards the conditions on which they depend.
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