PHYSIOLOGICAL 325 



movements of leaves at nightfalL Often, however, the stimulus is 

 some contact, as in the Sensitive Plant and Venus's Fly-trap. On 

 the other hand, there are similar movements (autonomic as dis- 

 tinguished from paratonic) for which it is not at present possible to 

 suggest any particular stimulus. They are sometimes called spon- 

 taneous, and the most striking instance is that of the Indian Tele- 

 graph Plant {Desmodium gyrans). The tripartite leaf has a median 

 leaflet, which is depressed and raised again in response to stimula- 

 tion, and two minute basal leaflets, which exhibit spontaneous 

 movements, somewhat semaphore-like, sometimes uniform, but 

 usually jerky. They fall rather more rapidly than they rise, and a 

 complete double movement takes 2-4 minutes. "As the turgor 

 variations tend to fluctuate regularly to right and left of the vertical 

 plane, the tip of each leaflet describes a narrow ellipse." The move- 

 ments cease in adverse conditions, but their significance is unknown. 

 It is probable that there are many similar autonomic movements 

 which are on so small a scale that they remain unnoticed. 



Summary. — ^Apart from the locomotion of entire plants, such as 

 some simple Algae, and of germ-cells, such as the male elements of 

 ferns, we may distinguish among plants (a) the all-round growth- 

 movements, such as nutation, {b) the movements in one direction, 

 induced by differential growth, provoked by stimuli acting on 

 a given internal structure and state of activity, e.g. heliotropic 

 bending to or from the light ; (c) turgor movements, usually localised 

 in special organs, induced by differential permeability of cell 

 walls, and provoked by stimuli, as in the Sensitive Plant; and 

 {d) autonomic movements, apart from recognisable stimuli, as in 

 the Telegraph Plant. 



Are Plants Nervous? — ^As we have already emphasised, a 

 great step towards unity of outlook was marked by Claude 

 Bernard's Legons sur les Phenomenes de la Vie communs aux 

 Animaux et aux Vegetaux (1879). ^^ became clear that plants 

 and animals share a common life, though their detailed expressions 

 of it are very different. The beech-tree feeds and grows, digests 

 and breathes as really as does the squirrel on its branches. In 

 regard to none of the main functions except excretion is there any 

 essential difference. Moreover, many simple plants swim about 

 actively; growing shoots and roots have gently-swaying tips; leaves 

 rise and fall, flowers open and close, with the waxing and waning 

 light of day. The tendrils of climbers, the leaves of the Sensitive 

 Plant, the tentacles of the sundew, the blade of Venus's Fly-trap, 

 the stamens of the barberry, the stigma of the musk, and many other 

 plant structures exhibit exquisite sensitiveness. In the sense of 

 answering back to stimuli, Wvey feel. How far we have got from the 

 aphorism of Linnaeus: Lapides crescunt; vegetahilia crescunt et 

 vivunt: animalia crescunt et vivunt et sentiunt. The "nervousness" of 



