Interrelation of Plant and Animal Pathology 57 



of the functions of its component cells. We are not surprised, 

 therefore, to find that access to water, for example, is a prime 

 essential condition of life in both kingdoms. We may on first 

 reflection argue that ordinary land plants require much more of 

 it than the higher animals do. Further acquaintance with the facts, 

 however, shows that another factor intervenes to bring about this 

 apparently greater water requirement. The essential difference in 

 fundamental life requirements of the green plant and the higher 

 animal is that the plant is able to manufacture its own food through 

 the energy supplied by sunlight. In order to accomplish this end 

 the plant must provide for a relatively enormous access of air in 

 order to get its required carbon dioxide, because this gas exists 

 in but small quantity in the atmosphere. The plant thus has to 

 open a large portion of its body and expose internal cells to the air. 

 This subjects these cells also to water loss by evaporation which 

 necessitates a constant current of water through the plant. Another 

 factor tending toward the same result is the necessity for a large 

 body surface exposed to the incident rays of sunlight, in order to 

 secure the necessary energy for food manufacture. 



The process of food digestion is practically identical in the two 

 kingdoms. Both animals and plants require virtually the same 

 kinds of foods. Nothing could be more convincing in this direction 

 than that the two chief sources of bread for the human race, rice 

 and wheat, consist mainly of carbohydrate and protein food stored 

 up by the plant to nourish its young. The striking difference in 

 relation to food between animals and plants is that green plants are 

 able to manufacture their food from raw mineral materials, in other 

 words, are autotrophic. This function, restricted to chlorophyll 

 bearing plants, is special, and if we are to trust the conclusions of 

 Macchiati may go on outside the organism from purely chemical 

 solutions of chlorophyll in the presence of sunlight and carbon 

 dioxide. It is true, also, that a comparatively small proportion of 

 the cells of a higher plant contain chlorophyll, and the other cells 

 of the individual have to be nourished in exactly the same manner 

 as animal cells. 



In like manner we find the respiratory function identical in the 

 two kingdoms, in fact we may say that the liberation of energy by 

 oxidation of combustible materials is about as nearly a phenomenon 

 common to all living creatures as can be found. 



