106 LIFE IN NATUEE. 



mushrooms have been known to lift up heavy masses 

 by their growth, sometimes raising in a single night 

 a stone weighing many pounds. The forces which 

 produce growth operate with enormous power. And 

 well they may ; for they are essentially the same 

 forces those arising from the chemical properties 

 of bodies which in our own hands produce the 

 most powerful effects, and are often indeed so violent 

 in their action as to be wholly beyond our control. 

 But it is clear that such cases as this can offer no 

 difficulty in respect to the laws of growth. Every 

 one must see that the mushroom would certainly 

 not have raised the stone if that had not been the 

 direction in which its expansion was resisted least. 

 In this respect, the case seems precisely similar to 

 the expansion of steam in a boiler raising the piston. 

 The mechanical resistance yields when the invisible 

 inward force reaches a proportionate amount. 



There is, however, another class of instances to 

 which we must refer. These are the forms which 

 result not directly from growth but from decay, and 

 of which the spongy pith of many plants is an 

 example. The irregular cells and plates of which 



