The Living Machine 299 



another tears down, and so it goes, in the lives of the unseen 

 bacteria of the soil, as well as in the affairs of man. 



But while most animals and plants differ so 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 

 growth material obtained. And this food must be rendered 

 soluble and dialyzable in order that it may pass through 

 membranes which surround each cell, i. e., must be digested. 

 While in the higher animal there is a special place where 

 digestion and absorption occur (the digestive tract) and the 

 digestive ferments are formed by special glands (liver, pan- 

 creas, etc.), in the plant there is no such specialized tract or 

 glands for the functions of digestion and absorption, these 

 taking place generally in the leaves. There are however cer- 

 tain specialized tubes of cells in the root and stem which 

 taken together form "conducting paths," for the water, with 

 its dissolved salts ascending from the soil, and the sugar 

 descending from the leaves to root and stem, there to be stored 

 as starch for future use. And after digestion the food must 

 circulate through the plant to all its parts, and be built up 

 into its tissues by constructive ferments analogous to those 

 of animals. 



In this circulation of water with its dissolved substances 

 through root and stem we see one of those marvelous, and 

 as yet inexplicable phenomena of life, which have caused 

 so many biologists to throw up their hands in despair and 

 ascribe to life some occult power undiscoverable by the sci- 

 entific methods of the physicist and chemist. 



From the leafy surface of humblest herb and mightiest tree, 

 transpiration takes place, or the loss of water absorbed by the 

 roots from the soil. During the day this water is usually 

 quickly evaporated, but in the cooler air of night evaporation 

 is reduced and some of the transpired water remains as dew 

 upon the leaf. The pressure lifting the water from the soil 

 to the leaf may be as great in some cases as that which would 

 be exerted on the earth's surface by an atmosphere six to 

 eight times the thickness of the present one, a pressure suffi- 

 cient to support a column of water between two and three 

 hundred feet high. 



Various attempts have been made to explain the rise of 

 sap in plants but as yet with no great success. The evapora- 

 tion from the leaves and the absorption of water by the cells 

 are the principal factors claimed as causing this wonderful 

 phenomenon. Neither factor however is adequate, and the 

 best we can do here, as in so many other cases, is to confess 

 our ignorance, and press onward in the search for knowledge. 



