674 LECTURE XXXVIII. 



cause in both cases may be essentially the same ; as in the case of a tendril which is 

 still straight but actively growing in length, so also in the case of the shoots of 

 twining plants, so long as they are still elongating, the tendency to make spiral 

 coils exists, but the tendency is not exhibited, or only to an insignificant extent, 

 simply in consequence of the rapid growth. If the growth is enfeebled, however, and 

 approaches complete extinction, the side which will be concave on coiling first ceases 

 to grow entirely, while the opposite one continues to elongate for some time longer 

 as the growing side. It thus happens, and not very seldom, that even long pendent 

 shoots, which have found no support and therefore have their growth interfered with, 

 finally make several corkscrew-like turns, and then die off altogether, as I have often 

 observed in the case of Dioscorea batatiis and D. Japonica. But it happens much 

 more frequently that feeble shoots before they cease to grow altogether, first give up 

 their circular nutation, suddenly erect themselves, and then, in the course of several 

 days, make 2-5 corkscrew-like, and usually very narrow fiat coils, and then cease 

 growing altogether. In this case there are evidently two factors working together 

 to the same end ; on the one hand, the tendency already referred to, resembling that of 

 tendrils, to roll themselves up spirally ; and, on the other, the vertical position due to 

 geotropic erection, and which even in vigorously growing shoots induces the forma- 

 tion of free coils. 



It will, however, certainly need further and very careful researches to derive the 

 mechanical theory of twining from the statements just made, and which I put forward 

 simply as facts. 



It is a question as yet undecided whether twining shoot-axes are irritable. The 

 question was answered in the affirmative on Insufficient grounds by Mohl (1827) to 

 whom we owe the first useful investigation of twining plants; but subsequently 

 denied on still less sufficient grounds by Darwin. From De Vries' researches the 

 question also appeared to me to be decided in the negative ; but a more careful 

 apprehension of the term irritability gives the matter another aspect. When Darwin 

 denies irritability to twining plants because they make no twining movements when 

 slightly pressed or rubbed, it is much as if sensitiveness to light were denied to 

 the retina of the eye because mere rubbing of the eye-lid does not produce vision. 

 The better reasons which De Vries adduces against the irritability cannot be so 

 quickly disposed of, though in my opinion they cannot be maintained. 



It is above all important what is meant by the word irritability. I understand 

 by this term, as already said, that kind of reaction which the living organism ex- 

 clusively, as such and in consequence of its vital capacity, exhibits towards external 

 influences. When it is found then that an inverted twining plant spontaneously 

 uncoils itself again from the support, when the apex coiled round a support and 

 artificially uncoiled spontaneously straightens itself and nutates, when merely placing 

 the shoot erect causes it to make spiral turns as if it had a support — I find in all 

 this the essential characteristics of irritability in the above sense, though of course it 

 affords no explanation of the true mechanism of twining. 



The irritabiUty of twining shoot-axes, however, comes in not merely as one of the 

 causes of twining. I have for many years observed thousands of twining plants 

 throughout their lives, and found that vigorous shoots when they grow out beyond 

 the support or meet with none at all, become moribund; it is easy to observe 



