488 Minnesota Plant Life. 



tions in their relative positions and combinations might be al- 

 most infinite. Certainly, however, the difference between one 

 piece of living substance, such as the egg of a fish, and another, 

 such as the egg of a fern, consists less in chemical composition 

 than in physical structure. Chemically, all living substance lies 

 within limits which have been just outlined. 



Physiological character of living substance. The following 

 are the most important physiological characters of protoplasm : 

 i. Assimilation, the power of initiating and maintaining com- 

 plex chemical changes by which not-living matter is brought 

 into the living condition and into relation with the living sub- 

 stance. 2. Grozvth, a term here applied to the increase of living 

 substance in mass. 3. Irritability, the quality of responding, 

 after a manner determined by heredity, to impulses originating 

 within or without the body. 4. Reproduction, the power of 

 separating, from the body, portions of living substance that, 

 under the influence of heredity, may recapitulate the develop- 

 mental stages of the parent. Together with assimilation, in its 

 broad sense, goes on a variety of chemical processes. By these, 

 waste products are formed and excreted, accessory products 

 are combined and modified, and a variety of complex substances 

 are broken down into simpler forms, thus liberating energy that 

 is either used in growth and movement or reappears as body- 

 temperature, phosphorescence, electrical disturbances or some 

 form of mechanical work. 



Requisitions made by living substance on its environment. 

 In order to maintain its physiological processes, living sub- 

 stance must have a supply of matter and a supply of energy. 

 The supply of matter, or the food of the organism, may be of 

 great variety, provided that it contain the necessary chemical 

 elements in assimilable form. The great primal source of 

 energy is the sun and upon its light and heat all living things, 

 in the final analysis, will be found depending. Some, however, 

 like the fungi and the animals receive much of this energy indi- 

 rectly. It is utilized directly by the green plants, the color of 

 which indicates the presence in their tissues of that extraordi- 

 nary accessory product of living substance leaf-green, or chlor- 

 ophyll. This leaf-green that stains either the entire living sub- 

 stance of the cells that are set aside for starch-making, or cer- 



