i88 THE LIFE OF THE PLANT 



both the reason why the stem rises and at the same 

 time the reason why the same thing does not happen to 

 the root. The desired explanation must consist in a 

 difference of structure of some kind between the stem 

 and the root, because we cannot admit that one and 

 the same force will act on exactly similar bodies in a 

 different way. 



Let us see what explanation we can find for the growth 

 upwards of stems. In order to do this we must begin 

 by studying a curious property of vegetable organs, a 

 phenomenon known as the tension of tissues. Let us 

 cut a long piece from the middle of a young growing 

 stem, as is shown in fig. 53, p, where the shaded part 

 represents the epidermis and cortex, and let us moisten 

 it with water to prevent it from drying up ; then let us 

 split it longitudinally into two halves with a sharp knife. 

 Both parts will immediately curve as in fig. 53, r. This 

 curvature can only be caused either by the outer side 

 of each part becoming shorter or the inner side longer 

 than before, or both at the same time. At all events we 

 come to the conclusion that in the undivided section 

 the external and internal parts are in a mutually 

 strained state ; the one stretches the other, and is itself 

 restrained in its tendency to elongate by the resistance 

 the other offers to extension. We can prove this fact 

 by making two slits instead of one, and separating 

 the external tissue in two parts and disengaging from 

 between them the middle tissues (fig. 53, s). We shall 

 now actually see that the middle part will stretch 

 and become longer than it was before at p, while the 

 external parts will shrink and become shorter than 

 they were at ^. It is clear that the inner parts of 

 the stem tend to elongate, but being opposed in this 

 tendency by the external parts stretch these instead. 

 This mutual tension of tissues plays a very important 

 part in the life of a plant ; it is to this property 

 that delicate and succulent stems owe their rigidity. 



