5i6 



LECTURE XIX. 



may exhibit different torsion, some being homodromously 

 and others antidromously twisted. Torsion is then clearly 

 not the cause, nor is it a necessary accompaniment, of 

 twining, but it arises in twining stems from causes most of 

 which also produce it in stems which do not twine, the only 

 cause peculiar to twining plants being the friction against 

 the support. 



We have now to consider the direction of twining. It 

 was pointed out in a previous lecture (p. 363), that whereas 

 some stems circumnutate in the direction of the sun, others 

 circumnutate in the opposite direction ; and since, as we 

 have seen, the direction of twining of a stem is the same 

 as that of its circumnutation, there are two modes of twining 

 as there are of circumnutation. The direction of twining 



Fig. 54 (after Payer). 



A, Hop twining with the sun; B, Convolvulus twining 

 against the sun. 



