780 MECHANICAL LAWS OF GROWTH. 



at least this seems the only explanation of the contraction of the surface in contact 

 in the case of tendrils the growth of which has already become slow. We have 

 however as yet no knowledge of the mode in which the slight pressure of a light 

 thread or that of the revolving tendril on a support causes this alteration of growth 

 not only at the point of contact, but along the entire tendril. 



The only cause of the spontaneous coiling of tendrils when not fixed to a sup- 

 port is that the upper surface continues to lengthen for a considerable time after 

 the growth of the under surface has ceased. The cells of the growing upper sur- 

 face probably withdraw from those of the under surface a portion of their water (as 

 the inner layers of the pith from the outer layers, see p. 725), which causes the 

 latter to become shorter, and the former to become longer. 



Without entering further into the numerous questions of a purely mechanical 

 character connected with the curving of tendrils, it may at least be explained why 

 thick tendrils are unable to twine round very slender supports. If two tendrils are 

 compared one of which twines round a slender, the other round a thicker support, it 

 will be seen that in the former the proportional difference in length of the outer and 

 inner sides must be greater than in the latter. If a thick and a slender tendril twining 

 round supports of equal thickness are compared, the proportionate difference in 

 length of the outer and inner surfaces will be greater in the former than the latter case ; 

 and if the support is supposed to decrease constantly in thickness, the difference will 

 increas©^nlore rapidly in the case of the thick than in that of the slender tendril, and 

 the qiU'festion arises whether the difference in growth of the two surfaces of the tendril 

 can reach to any given amount or not. The difference in length between the two 

 surfaces caused by unequal growth has, in fact, a limit, as is shown by experiment. 

 The slender tendrils of Passiflora gracilis twine firmly round threads of silk ; the 

 thick tendrils of the vine on the other hand twine only round supports which are at 

 least from 2 to 3 mm. thick. The most strongly curved tendril of a vine which 

 I could find had twined firmly round a support 3-5 mm. thick, and in a nearly circular 

 coil ; the mean thickness of the tendril at this spot was 3 mm. The concave surface 

 of a coil was nearly 11 mm., the convex outer surface nearly 29 mm. long, the 

 proportionate length of the two surfaces therefore nearly as i : 2"6. If this tendril 

 3 mm. thick were forced to twine round a support only 0-5 mm, in thickness, an 

 almost circular coil would have on the concave surface a length of i-6 mm., on the 

 convex surface a length of 20*4 mm. ; the relative length of the two surfaces would 

 therefore be as 1:13; and it does not seem possible for growth to cause so great 

 a difference in length between the two surfaces of a tendril. If, on the other hand, 

 the problem were to cause a tendril 0*5 mm. thick to twine firmly round a support 

 of the same thickness in nearly circular coils, it would only be necessary that 

 the inside of a coil should be i-6mm., the outside 47 mm. long, or that the pro- 

 portion between the two surfaces should be as 1:3. 



In order for a tendril to attach itself firmly to a support, it is not sufficient that 

 its coils should merely be in contact with it ; they must be firmly adpressed to it. 

 That this is actually the case is seen when a tendril is made to twine round a smooth 

 support, and the support is then withdrawn ; when, as de Vries has shown, the coils 

 become at once closer and increase in number. This fact shows also that a tendril 

 which is irritated by contact with a support endeavours to form coils the radius of 



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