496 



TRANSFORMATION OF ENERGY 



point, more especially as to whether they may be compared with genuine tendrils 

 so far as the mechanics of curvature and their irritability are concerned. Into 

 this question we need not enter. 



A brief reference may now be made to Cuscuta. This remarkable plant 

 is of special interest as forming an intermediate link between tendril-bearers and 

 twiners. According to Peirce' s researches (1894), Cuscuta has two regularly alter- 

 nating conditions ; in the first it twines in a left-handed manner and, as a result 

 of rotatory movements, the shoot apex forms a number of steep spirals round some 

 vertical support only. After a time the second phase sets in, during which the 

 plant behaves like a tendril, twining round its support in much less steep and 

 tighter coils. These coilings are induced by contact with solids but not with 

 moist gelatine, so that the sensitivity of the stem is obviously quite comparable 

 to that exhibited by tendrils. In contrast, however, to tendrils, Cuscuta, during 

 the time when it is sensitive to contact, is also geotropically active as well, and 

 hence twines round vertical supports only. It may be noted also in passing 

 that Cuscuta is capable of forming haustoria as a consequence of contact 

 stimulus. 



Haptotropic movements are also manifested freely by carnivorous plants, 



Fig. 156. Leaves of Drosera roiutidi/olia^ that on the left viewed 

 from above, that on the right viewed from the side. Enlarged. After 

 Darwin, from the Bonn Textbook. 



although in their case the biological significance of 

 the movement is entirely distinct from that of 

 tendrils ; they enable the plant to catch and digest 

 small animals. Unfortunately we are still without 

 an up-to-date and comprehensive memoir on the 

 nature of the movements in carnivorous plants, so that our knowledge on many 

 important questions is very incomplete. We shall confine ourselves exclusively 

 to the study of the leaves of Drosera (Darwin, 1876 b). In the common native 

 species, Drosera rotundifolia, the leaves are almost circular, attached to the axis 

 by long petioles. The upper side of the blade is somewhat concave and studded 

 with tentacles. The tentacles in the middle of the blade are glandular, 

 shortly stalked and stand erect, while those at the periphery are long stalked 

 and bent outwards. Each tentacle is tipped by a drop of sticky secretion which 

 sparkles in the sun like a dewdrop — hence the popular name of the plant, ' sun- 

 dew.' Small insects which chance to alight on the leaf are held fast by the sticky 

 secretion, and by further excretion from the glands digestion of the body is 

 effected. (Compare p. 184.) When a single tentacle has been touched by an 

 insect, not only that one, but other tentacles also exhibit curvatures, until finally 

 nearly all the glands come in contact with the prey and aid in digesting it. Let us 

 assume that the insect touches one or more of the shorter tentacles in the centre 

 of the lamina, these we shall find remain erect, but the stimulus is transmitted 

 from them outwards, so as to induce an inward radial curvature of every peri- 

 pheral tentacle. If the insect be caught by a peripheral tentacle, the latter only at 



