192 THE LIFE OF THE PLANT 



of an organ grows more quickly. We do this by mak- 

 ing the points of contact of the india-rubber rods with 

 the discs lie this time near their lower edge instead 

 of at their centres (fig. 54, H). On pushing in the 

 handle as before, we notice quite a different result ; 

 while the lower spiral elongates in a straight line, or 

 even hangs down a little {d') under its own weight, the 

 upper one curves upwards (c') more or less sharply. 

 This result is easily explained by the construction of 

 the model. The springs are chosen purposely of varying 

 elasticity : the upper one of much thicker wire exerts 

 a stronger resistance to the movement of the bar than 

 the lower one of thinner wire. We infer that the un- 

 equal, unsymmetrical pressure manifests itself in a visible 

 curve only when there is a certain degree of mutual 

 tension between the parts. Evidently the same can 

 be applied to growth. Unequal and unsymmetrical 

 growth will be followed by a sharp and perceptible 

 curvature of the organ only when this organ attains 

 a certain degree of turgidity owing to the mutual 

 tension of tissues. But does a young growing root 

 actually show such tension of tissues as we saw in the 

 stem ? A glance at such a root will convince us of 

 the absence of such tension inside it. If we hold a 

 stem horizontally it will not bend, nor droop, whereas 

 a root will frequently hang down like a stem already 

 withered. If we study the structure of the epidermis in 

 the stem and the root, we shall find a further difference 

 which has long attracted the attention of anatomists. 

 The epidermis of the stem consists of cells with thicker 

 walls, and moreover is covered with a special membrane 

 which it is difficult to moisten with water, and which 

 is verj' elastic ; on the other hand the epidermis of the 

 root consists of cells with thinner walls, it easily absorbs 

 water, and therefore is more easily stretched, and 

 less elastic than the epidermis of the stem. If now 

 we make a direct experiment similar to the one we 



