488 Minnesota Plant Life. 



tions in their relative positions and coniljinations might be al- 

 most inthiile. Ccrtainlv, 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 pln-sical structure. Chemically, all living substance lies 

 within liiuits which have l)cen just outlined. 



Physiological character of living substance. The following 

 are the most imjjortant ])liysiological characters of protoplasm: 

 1. Assiiuilatioii, 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. CroicfJi, a term here applied to the increase of living 

 substance in mass. 3. Irritabilih\ the quality of responding, 

 after a manner determined by heredity, to impulses originating 

 within or without the body. 4. Rcprodiiciioii, 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 \\ith 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 comj^lex 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, li\ing sub- 

 stance nuist ha\e a supply of matter and a sup])l\- of energy. 

 The supply of matter, or the food of the organism. ma\' l)e of 

 great variety, provided that it contain the necess:n'y chemical 

 elements in assimilable form. The great ])rimal source of 

 ciicrf^y is the sun and upon its light and heat all li\ing things, 

 in the final analysis, will be found dei)t'ndiiig. v'^onie. however, 

 like the fungi and the animals recei\-e mncli ot tlii> energy indi- 

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

 which indicates the presence in their tissues of th:it extraordi- 

 nary accessorx' ])roduct of fixing sub>tance — leaf-green, or chlor- 

 opJn'll. This leaf-green that stains either the entire living sub- 

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



