TWINING OF TENDRILS. 779 



growth, they take place only when the external conditions of growth are favourable, 

 and the more energetically the more favourable they are ; this is the case when food 

 is abundant, temperature high, and the plant contains abundance of sap, the result 

 of a copious supply of water combined with small loss by transpiration. Under 

 these conditions tendrils can, as I have shown, carry on their nutation and sensitive 

 movements even in the dark, and can twine and coil round supports. An instance 

 is afforded by plants of Cucurbita Pepo, the upper parts of which are grown in a 

 dark vessel, and which are nourished by green leaves exposed to light. 



As regards the mechanical conditions of the curvatures caused by contact, as 

 well as the coiling of free tendrils, it cannot be doubted that we have here to do 

 with processes of growth and of its alteration by transverse pressure on the side 

 which is growing less rapidly. The tendrils are only sensitive to contact or pres- 

 sure so long as they are in a growing state. A curvature due to irritation may be 

 effaced during growth, as for instance the curvature of growing shoots caused by 

 concussion ; but if the irritation from the support lasts for a longer time and a coiling 

 takes place, the difference in length between the convex and concave surfaces 

 remains permanent. The cells of the convex are longer than those of the concave 

 surface (as in roots turned downwards or nodes of Grasses turned upwards) ; in 

 thick tendrils which coil round slender supports the difference in length is so great 

 that it strikes the eye at once without measuring. De Vries's recent experiments 

 on tendrils that have not yet coiled, which he marked with transverse streaks and 

 measured after they had coiled, show that the growth of the convex surface 

 is more considerable, that of the concave surface less so than in the portions of 

 the same tendril above and below the curved part that have remained straight. 

 A tendril oi Cuciirhita Pepo twined round a support 1*2 mm. thick; after the curv- 

 ature was complete, the increment of the curved part for each millimetre of original 

 length was i'4 mm. on the convex surface, while on the concave surface it was only 

 OT mm.; the mean increment on both surfaces in the portion that remained straight 

 amounted to 0*2 mm. If the growth which takes place in the entire tendril at the time 

 of contact with a support is small, a considerable acceleration occurs on the convex 

 surface, but in general there is no elongation on the concave surface, or there may 

 even be a contraction ; in the case of a tendril of Cucurbita this contraction amounted 

 to nearly one-third of the original length. 



Similar alterations in the length of the convex and concave surfaces are observ- 

 able in the spontaneous coiling of free, as well as in the coiled portion of attached 

 tendrils betw^een the base and the point of attachment; and since in these cases 

 the amount of growth which takes place in the entire tendril is usually small a short 

 time previously, the contraction of the concave surface is, according to de Vries, a 

 very common phenomenon. 



The conclusion to be derived from these phenomena and from others not 

 described here is that the growth of the surface not in contact is first of all increased 

 by the pressure of the support; the support presses the surface that is in contact, 

 and the pressure which the concave surface undergoes arrests its growth, or even 

 causes a contraction in it. It seems probable that a relaxation of the parenchyma 

 of the surface in contact (by giving off water to the parenchyma of the upper sur- 

 face) and a consequent elastic contraction of its cell-walls, contribute to this result ; 



