CH. V] ELASTICITY. 137 



(158) Imperfect elasticity of plant-tissues. 



The fact that the tissues of a growing shoot or leaf are 

 extensible, but not perfectly elastic, can be demonstrated 

 on a variety of material, e.g. a flower scape of Polyanthus, 

 or the leaf of a Narcissus: the form of the last-named 

 makes it convenient for the purpose. For this and 

 similar experiments a strong sheet of cork mounted on a 

 board is convenient: one end of the leaf is clamped 

 between the mounted cork and a free block of cork, in 

 such a position that the other end of the leaf projects 

 beyond the board. Two marks about 100 mm. apart are 

 painted on the leaf, one being close to the clamped end. 

 The distance between the marks having been read on a 

 mm. scale (clamped to the cork-board) the projecting end 

 of the leaf is pulled with the hand ; the distance between 

 the marks is now to be read off without diminishing the 

 traction, and again when the leaf is left to itself. The 

 leaf will be found to be permanently extended ; the tem- 

 porary and permanent extensions should be recorded in 

 percentages of the original length. 



(159) Cyclometer 1 . 



Take a straight turgescent shoot, e.g. a young cabbage- 

 shoot, bend it forcibly, and then release it : it will be found 

 to have taken on a permanent curve. This is only another 

 way of demonstrating what is shown in experiment 158 : 

 the cortical tissues on the convex side of the shoot are 

 forcibly elongated by the bending, and being imperfectly 



1 Sachs' Text-book, p. 784. 



