THE STEM OF VASCULAR PLANTS. 139 



a high degree, this spiral torsion of the fibres of the 

 stem; it only commences at some distance above the 

 neck. 



Twining stems may be erect or prostrate in their 

 young state ; but after some time they elongate much, 

 and twist themselves spirally ; if they do not meet any 

 body capable of serving for their support, they fall again, 

 or sometimes twine round one another, so that some 

 individuals of the same species, or some branches of the 

 same individual, mutually support each other ; if they 

 meet a suitable support, they twine themselves round it, 

 in a direction constant in each species — from right to left, 

 as in the French Bean — from left to right, as in the Hop, 

 This direction is determined by supposing oneself placed 

 in the centre of the spire, with the stem turning round 

 his own body. The cause, whether physical or ana- 

 tomical, of the twining disposition of several stems, and 

 the peculiar direction which affects each, is perfectly un- 

 known. Some have thought that it is connected with the 

 daily course of the sun, and its action upon the vegeta- 

 tion. Although it would be very extraordinary for one 

 cause to produce opposite eifects, yet we must not reject 

 this opinion ; and the ingenious Dr. Wollaston conjec- 

 tures that it might be verified, if, by observing individuals 

 of the same species, with twining stems, in the two 

 hemispheres, it were carefully remarked whether they 

 turned in the same or in opposite directions ; — an obser- 

 vation which is very simple, and deserves to be recom- 

 mended to travellers in the southern hemisphere. 



xVlthough it may be the cause of it, I will add that in 

 several plants this disposition remains throughout their 

 whole lives, however more or less woody they may be ; 

 as, for example. Wisteria fruticosa and Periploca grceca. 

 There are, on the contrary, some plants where this ten- 

 dency is only visible in the young shoots, and disappears 



