2 THE LIVING PLANT 



itself or in special tissues, endosperm and perisperm : and 

 since the food is stored in a form mostly insoluble and non- 

 assimilable, water is the first essential and appropriate en- 

 zymes the second, for not before it is hydrolized can food be 

 translocated from its storage cells and passed by osmotic 

 processes to the active tissues. The enzymes may be elabo- 

 rated in the cells or tissues containing the food, or may be 

 secreted by specialized structures, the scutellum for example. 

 Often the products of hydrolysis may be recognized by simple 

 means, sugar for instance, in germinating barley : but some- 

 times their assimilation may be so rapid that identification is 

 difficult ; indeed, on occasion their presence can only be in- 

 ferred from the results of carefully controlled test-tube ex- 

 periments, glycerol, for example, in germinating Ricinus. 

 The embryo thus presented with appropriate food, grows and 

 develops. Growth is in some degree an understandable 

 problem which, on the present elementary occasion, can be 

 sufficiently indicated in a few words. Of necessity must a cell 

 be nourished through its surface and growth will take place 

 if assimilation be greater than waste by oxidative and kindred 

 processes. But growth means increase, and as this increase 

 in bulk takes place the surface area of the cell is proportion- 

 ally lessened. A stage ultimately will be reached when the 

 area of the surface is so limited in proportion to the volume of 

 the cell as to permit the entry of only sufficient food to make 

 good the losses ; thus the surface area is a limiting factor. 

 One of three things now is possible : the cell may remain as. 

 it is, a permanent tissue element ; it may develop further, 

 using up its own contents either entirely or in part in fitting 

 itself for another function, water transport, for instance ; or 

 it may divide and by so doing increase its surface area in 

 relation to its volume, in which event the cycle may restart. 

 Growth thus can be interpreted in terms of physical chemistry : 

 the first possibility mentioned hardly requires contemplation, 

 since nothing is easier to do than nothing. The third pro- 

 position is less easy to understand ; the second is a mystery. 

 '1 hus, why should the daughter of a merismatic cell develop 

 into a phloem element if it be cut off on the one side of its 



