402 READINGS IN BIOLOGICAL SCIENCE 



human body, including the as yet inscrutable processes of thought; all these 

 coming from an apparently simple cell a little more than one-hundredth 

 of an inch in size, is a wonder beside which the magic of an Aladdin or the 

 miracles of holy writ fade into ghostly paleness. The enthusiast in the ranks 

 of the mechanist has attempted to remove even this most distinctive feature 

 of living things, by showing that non-living matter may in a sense repro- 

 duce itself, as new crystals form in an evaporating salt solution. However 

 feeble such a comparison may be, it is nevertheless true that all phases of 

 reproduction are all intimately associated with physico-chemical changes 

 taking place in these cells. 



What of the mechanism whereby this wonderful machine of life utilizes 

 its fuel? Herein lies one of the fundamental differences between the living 

 and the non-living machine. Whereas the latter uses its fuel solely in the 

 conversion of potential energy into heat and work, the former, in addi- 

 tion to these two functions, also converts some of its fuel into its own sub- 

 stance to take the place of worn-out parts, and to build new parts and en- 

 large those already formed in development and growth. 



Turning from the world of animals to that of plants, we find in the latter 

 a parallel to all of the metabolic processes of the former. The average per- 

 son is accustomed to think of a plant in terms of the green thing which he 

 finds in garden, field or forest. But when we go a-hunting mushrooms, or 

 poke aside the rotting remains of a fallen tree, we discover other plants 

 which live a different sort of life from that of the tree or shrub or herb. 

 And should we delve yet further into Nature's recesses, and penetrate that 

 hidden world to which the microscope gives entrance, we should discover 

 creatures concerning whom no one can say whether they are plant or ani- 

 mal. Some of these uncertain forms are claimed by both botanist and 

 zoologist as belonging to their own special field of study, for in some re- 

 spects they are distinctly animal, in others plant in nature. 



Perhaps the most fundamental difference between the higher plants and 

 animals is in their metabolism. While the latter are spenders, the former 

 are hoarders of energy, taking raw materials, carbon dioxide from the air 

 and water from the soil, and from these constructing by the energy of the 

 sun, acting through the green chlorophyll of leaf and stem, their own 

 food stuffs; thereby converting the radiant energy of sunlight into the 

 chemical energy of sugar and of starch. From the soil and the air the plant 

 obtains its nitrogen and from the soil the other inorganic substances which 

 are used to build its protoplasm, and combining these with the sugar it 

 builds up its protoplasm. Someday, perchance, the chemist, imitating na- 

 ture will learn to make our starch and sugar for us, and bid defiance to the 

 "man with the hoe." 



But while most plants differ widely in their metabolism, fundamentally 

 their ways of life are alike. Both must have food, from the combustion of 

 which their energy is derived, and from which their wastage is replaced and 



