386 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Another but a totally different use for this crank-action seems to 

 be seen in the poplar, when forming a gall for a certain insect. This 

 insect punctures and lays an egg in the flat middle part of the petiole 

 of a leaf; and to judge by the figures given, it would appear that 

 this part of the petiole revolves twice. The two closely-depressed coils 

 swell up to make the gall, so that it would seem both from this and Mr. 

 Shannon's experiment with a freely hanging pencil, which could give 

 no " purchase " to the tendril, that to be fixed at both ends is not 

 necessary in order to produce the rotatory motion. 



The Breaking Strength of Tendrils. — Mr. W. D. Brush, in writing on 

 this matter,* says that tendrils exposed to tension, and also having 



formed contact v/ith a support, had a 

 much higher breaking strength (1007 

 grammes average) as compared with 

 tendrils in contact only (651 grammes) 

 and free tendrils (190 grammes). 



Tension certainly increases strength 

 of Tendrils, sometimes by as much as 

 50 per cent, in the middle third of the 

 length. By radial pressure (obtained 

 by a mercury column in an india-rubber 

 tube enclosed b}^ the tendril), an in- 

 creased breaking strength was obtained 

 (990 grammes as compared v/ith 727 

 grammes). Contact, pressure, and ten- 

 sion all increase the breaking strength. 



Responsiveness to Force. — The ques- 

 tion has been raised v»^hether these 

 mechanical adaptations arise in conse- 

 quence of, i.e. in response to, various 

 pressures, strains, &c. This was Darwin's view, for he describes 

 and illustrates several cases of chmbing plants, as Bryony, Solanum 

 jasminoides, vine and Ampslopsis. In all cases, the climbing organ 

 alters its structure only after having caught some object.! 



Mr. Herbert Spencer found from Mr. Croucher's experiments 

 with Cactus that it developed wood where it was subjected to artificial 

 strains ; but when it was tied up no formation of wood took plaxcj 

 This corresponds with my own experience with cucumbers trained 

 against a wall. The stems and shoots being tied to horizontal wires, 

 the tendrils entirely refused to act. They would neither cling b}^ the 

 terminal hooks nor form coils at all. 



This loss of the power to cHmb without the stimulus is not 

 uncommon. Mr. Hemsley, describing Sphacnocodo7i ohtiisifolinm, a 

 native of tropical Africa, where it has a dwarf habit, and is sub-erect 



* Bot. Gaz. June 1912, pp. 453-477. 

 f Climbing Plants. 



% On the Circulation and the Formation of Wood in Plants," Trans. Linn 

 Soc. XV. p. 405, 1866, 



