490 LECTURE XIX. 



i : 2'6. Supposing now, the tendril had to curve round a support 

 o'5 mm. thick; the length of the concave side would be r6 mm., and 

 that of the convex side 20-4 mm., and the proportion would be 1:13. 

 It seems that such a great difference in length cannot possibly be at- 

 tained. 



It is for the same reason that relatively thick tendrils do 

 not apply themselves closely to the sides of flattened supports, 

 but only touch the projecting angles, whereas thinner tendrils 

 apply themselves to the flat sides, for they are capable of 

 curving sharply round the angles. Von Mohl observed tendrils 

 of Lathyrus odoratus and of Pisum sativum twining closely 

 round strips of tin. 



If the support is so thick that the tendril is not long enough 

 to make one turn round it, it is impossible for the tendril to 

 twine round it. When the support is rather thick, but not 

 so as to prevent twining, the coils formed around it by the 

 tendril are not even, but present -undulations. De Vries attri- 

 butes this to the fact that the general tendency of tendrils to 

 make coils smaller than the diameter of the support is very 

 strongly marked in this case. Small lengths of the tendril are 

 in consequence made to bulge away from the support, and 

 subsequently apply themselves to it again, the displacements 

 causing the tendril to become wavy. 



According to von Mohl's observations, the position and 

 the nature of the support do not in any way affect the twining 

 of tendrils around it. Darwin has made some very remarkable 

 observations with regard to the latter point. He found, 

 namely, in the case of Echinocystis lobata and of Passiflora 

 graciliS) that a tendril is not stimulated by contact with another 

 tendril. He observed a singular fact in Echinocystis lobata. 

 In this plant the tendril forms a very acute angle with the 

 projecting extremity of the stem or shoot bearing it, and the 

 tendril would, were it to perform its circumnutation without 

 deviation, probably come into contact with the end of the shoot. 

 In order to avoid this, as it were, it becomes stiff and straight 

 as it passes near the shoot. The advantages which these 

 singular adaptations secure for the plants which possess them 

 are obvious : they may be summed up by saying that a, waste 



