488 



TRANSFORMATION OF ENERGY 



branches or leaves without laminae, it may be said that support is effected by- 

 means of lateral organs while the chief shoot grows straight on. In this respect 

 tendril-bearers differ markedly from most twining plants ; but there is another 

 and more important distinction, viz, that the twining stem can hold on from 

 below upwards only to a more or less erect support and twine in a definite 

 direction, i.e. to right or left, while tendrils can attach themselves also to 

 horizontal supports and can twist round them to the right or to the left, upwards 

 or dow^n wards. This points to quite different physiological properties of these 

 two closely related biological groups of plants — a point which will come out with 

 perfect clearness in the following treatment of the subject. 



Starting with typical tendrils, such as we find in the Leguminosae, Cucur- 

 bitaceae, or Passifioraceae, we find them to be long, slender, flexible structures, 

 which, as in Passiflora, arise singly in the axils of the leaves, or which, as in 

 the Cucurbitaceae, arise singly or in greater numbers on a tendril-bearer, 

 alongside a leaf on the chief axis (Fig. 153. Compare Goebel, Organographie, 



Fig. 153. Tendril-bearer of Sicyos aitgiilaius. b, feebly stimulated ; c, strongly stimulated; d, a. tendril which 

 has attached itself to a support. At W a. reversion of the direction of twining has taken place. From Dethbr'S 

 Smaller Practical Physiology. 



p. 610) ; in the Leguminosae the tendrils generally occur at the ends of leaves. 

 A transverse section of a tendril is, as a rule, circular, but often it varies from 

 that shape and becomes flattened. Commonly the anatomical structure is 

 markedly dorsiventral (O. Muller, 1887) ; it is possible to distinguish an upper 

 and an under side and a right and a left flank. Even if, anatomically, there be 

 no difference between the sides it is possible to recognize dorsiventrality by other 

 evidence, for very frequently the tendrils, as they develop from the bud, are 

 rolled up in a spiral manner and the convex side is then always the under side. 

 They begin to grow rapidly and in a few days reach their definite length by 

 straightening. During this period they perform peculiar movements, i. e. of cir- 

 cumnutation, which recall those of the twining plants, but which are of a purely 

 autonomous character and which, for that reason, cannot be discussed in this 

 connexion (Lecture XLI). It may be noted only that the tendril, in consequence 

 of the fact that one side is growing more rapidly than the other, is slightly bent 



