THE CYCLE OF LIFE 395 



are rare, and one of the reasons for this is that animals live 

 much more nearly up to their income than plants do. 

 It sometimes happens, one must admit, that an organism 

 grows larger for a time without taking in any food we can 

 see that in the growth of salmon-fry before they begin to 

 eat but what happens in such cases is a change of con- 

 densed stored substances into more dilute and bulkier form. 

 The embryo is cashing and re-investing its legacy of yolk. 

 The same is true of the shoots of a potato, sprouting in a 

 dark cellar ; they show true growth though the organism 

 as a whole is actually losing water in transpiration, and, 

 as its respiration shows, breaking down carbon-compounds. 

 What was stored in the tuber is being transformed. 



More difficult, perhaps, is the case of a young tadpole, 

 for careful measurements and weighings show that during 

 the period of most rapid growth, the weight of dry sub- 

 stance does not increase at all. During this period, it 

 seems, the imbibition of water is more important than the 

 assimilation of food-material. 



Plenty of assimilated food is the sine qua non of growth, 

 but the conditions imply appropriate environment along 

 other lines. Growth, like development, has its optimum 

 environment, but this differs so much for different organisms, 

 that it is difficult to make general statements in regard 

 to the agencies that favour and those that hinder growing. 

 What is one organism's meat is another organism's poison. 

 In a general way, it might be said, light is essential for the 

 growth of plants, for the assimilatory process of building 

 up carbon compounds is a photo-synthesis dependent on 

 the sunlight. But when we look into the matter more 

 carefully we find that, other things being equal, plants 

 grow more rapidly during the night than during the day. 



