GENERAL PHENOMENA OF LIFE 31 



the fungi and allied plants form, therefore, an intermediate group 

 between the green plants and the animals, i.e., between those entities 

 of living substance which assimilate the carbon from carbon dioxid 

 under the influence of the rays of the sun and those which derive their 

 energetics from foodstuffs. 



Assimilation implies that the organisms must ingest nutritive 

 material which, after its digestion, is absorbed and utilized. The 

 manner in which this ingestion is accomplished differs materially with 

 the general form and behavior of the organisms. In the case of free- 

 living and naked unicellular masses, the acquisition of the nutritive 

 substances takes place apparently at any point of the surface by the 

 process of engolfing, while in the more specialized organisms, it occurs 

 in a particular place, namely at the gullet. The reduction or digestion 

 of the food is then effected by means of enzymes contained in secre- 

 tions which hydrolyze it and render it dialyzable and assimilable. 

 But while many cells possess the power of digesting the foodstuffs 

 themselves, many do not. The latter, therefore, require already 

 prepared food. In the higher forms this preparation is effected by 

 special groups of cells forming the digestive organs. Foi* this reason, 

 we speak of intracellular and extracellular digestion. 



The phenomena of dissimilation are ushered in by the decomposi- 

 tion of the protoplasm, in consequence of which the various forms of 

 energy are then liberated. It is necessary, however, to form the ma- 

 terial lost anew, otherwise the catabolism might progress beyond a 

 certain limit and endanger the life of the cell. Clearly, oxygen is a 

 necessary factor in this reduction, at least in most organisms, but it 

 has not been definitely settled as yet whether it forms a true anabolic 

 product of the cell in the shape of '' intramolecular" oxygen, or whether 

 it is present in the surrounding medium in its molecular form to be 

 made use of as such whenever required. As a result of this oxida- 

 tion, the cell gives rise to a number of products which are of no further 

 use to it and are later on gotten rid of by the process of excretion. 

 These waste materials are of many kinds. Chief among them are those 

 arising from carbon and hydrogen, namely carbon dioxid and water. 

 A number of them are derived from the proteids, for example, urea, 

 uric acid, hippuric acid, creatin, etc., which are either suspended or 

 dissolved in water. Their complete reduction frequently requires 

 special agents which are brought to bear upon them through the media 

 of the excretions. 



The purpose of metabolism is to keep the cells in a physiological 

 condition, as evinced by the amount of energy liberated by them. 

 The cell, therefore, is the seat of life. It receives certain substances 

 and with them a definite amount of potential energy which is then 

 transformed into kinetic energy in its various forms. Thus, cells are 

 destined to produce work, either directly or indirectly. The green 

 plants, for example, may be regarded very largely as potential factors, 

 because their energy must first be produced in the presence of sunlight. 



