ELEMENTARY VITAL PHENOMENA 177 



excretions or as substances that possess still further importance in 

 the life of the organisms in question. 



If, now, the facts of metabolism be co-ordinated, it is found that 

 from the entrance of substances into the living cell to their exit 

 from it, metabolism consists of a long series of com]Dlicated chemical 

 processes which can be represented in the form of a curve with an 

 ascending and a descending limb. The ascending limb comprises 

 all processes that lead to the construction of living substance; the 

 apex is formed by the synthesis of the most complex organic 

 compounds, the proteids; the descending limb comprises the 

 processes of the destruction of living substance into its simplest com- 

 pounds. The beginning and end of the curve, i.e., the substances 

 that enter into and go out from the organism, are best known ; the 

 components that lie at the apex of the curve are known least, and 

 in large part not at all. 



The green plant-cell, even the simple, unicellular, green alga, 

 such as Protococctis, is a chemical laboratory in which, out of the 

 simplest inorganic materials — carbonic acid, water, and salts — 

 organic substance is manufactured, analytic processes and syntheses 

 going on hand in hand in the process. First, starch appears. 

 Starch with the help of nitrogenous salts serves to construct 

 proteids, in which process very various kinds of by-products arise. 

 But the green plant-cell does not complete this gradual con- 

 struction of proteids for itself alone, it does it at the same time 

 for all animal-cells, which in the course of evolution have lost 

 the power of manufacturing organic material out of inorganic. 

 The organic substances produced by plants serve as food for 

 herbivora, the flesh of herbivora as food for carnivora. Carnivora 

 can live upon proteid food alone. Hence it is seen that of the 

 substances that appear in metabolism some, as in plants, lead to 

 the construction of proteid, and some, as in carnivora, are derived 

 from the transformation of proteid. But in plants as well as in 

 animals a constant decomposition of proteid finally takes place, 

 and there result again, as definitive end-products of metabolism, 

 simple inorganic compounds, essentially the same materials with 

 which the construction of living substance was carried on, namely, 

 carbonic acid, water, and nitrogenous salts. All metabolism, 

 therefore, is merely a series of processes which are related to the 

 construction and destruction of proteids and their compounds. 

 This is true as well of the plant as of the animal. 



II. The Phenomena of Form-changes 



The form of organisms is not unchangeable. Apart from the 

 changes that are associated with motion, and which will be con- 

 sidered elsewhere, organisms show profound changes of form that 



