THE IRRITADILITV OF TENDRILS. 665 



llic increment on the curved portion for each mm. of tlic original length amounted 

 to 1-4 mm. on the convex side, but only o-i mm. on the concave side. The 

 mean increment on the portion which remained straight on both sides amounted 

 to 0-2 mm. If the growth of the whole tendril at the time of contact with a 

 support is but small, a considerable increase of growth in length is found to 

 occur on the convex side ; but on the concave side there is generally no elongation 

 at all, or there may be even a contraction ; in the case of a tendril of the Gourd this 

 shortening amounted to almost a third of the original length. 



Similar alterations in the length of the convex and concave sides are to be 

 observed in the spontaneous coiling up of tendrils, as well as in the coiled up 

 portions lying between the support and the base of attached tendrils; since in 

 these cases the growth of the whole tendril is usually slight shortly before, the 

 contraction of the concave side is also a very general phenomenon. (De Vries.) 



The whole of these phenomena and others not here described lead to the 

 conclusion that the growth in length of the untouched side is increased by the 

 pressure of the support; this forces over the side which is in contact, and in the 

 curvature which now follows the concave side is compressed and retarded in growth, 

 or even shortened. It seems to me probable that at the same time a relaxation 

 of the parenchyma of the touched side (due to its giving off water to the parenchyma 

 of the upper side), and a corresponding elastic contraction of its cell-walls co-operate 

 here; at least, in the case of tendrils which are growing slowly, the shortening 

 of the side which is in contact appears explicable in no other way. But how the 

 slight pressure of a light thread or the pressure of the nutating tendril on a support 

 effects these alterations in the growth, not only at the parts in contact but along the 

 whole tendril, remains for the time being entirely unknown. 



The spontaneous coiling up of tendrils which are not fixed to supports is 

 probably only due to the fact that the upper side goes on elongating for some time 

 after the lower side has already ceased to grow ; the cells of the growing upper side 

 probably withdraw a portion of their water from those of the lower side (as the inner 

 layers of pith do from the outer layers, cf. p. 573), whence the latter become shorter, 

 while the former elongate. 



According to later investigations by De Vries, the first direcdy perceptible effect 

 of a stimulus consists in the increase of turgescence on the free untouched upper side 

 of the tendril, and it is directly in consequence of this that the growth also of this 

 side is accelerated. 



The real problem here again, therefore, as in the case of the motile organs 

 of leaves sensitive to light and contact, is why the conditions of turgescence 

 on opposite sides of the organ are modified by the stimulus, and here again we 

 shall point out that it is evidently the protoplasm which in the first place receives the 

 stimulus, and then, by alteration of its molecular condition causes the turgescence 

 of the cells to change. In principle, we may say, the stimulation in the case of a 

 tendril is the same as in the case of a motile organ of Mimosa^ only that it is here a 

 matter of a continuous though very slight contact, and the alteration of turgescence 

 leads to a permanent change by means of growth. 



Without going further into the numerous questions of a purely mecha- 

 nical nature connected with the curvatures of tendrils, it need here simply be 



