ELEMENTARY VITAL PHENOMENA 171 



2. Secretions and Excretions 



It is neither necessary .nor possible to examine here in detail 

 the whole series of secretions and excretions which plant and 

 animal cells afford in their metabolism ; our consideration shall 

 therefore, be limited to the most important of these. 



a. Secretions 



Since it is characteristic of secretions to be of use to the 

 organism, it is easy to understand that many secretions remain 

 continually within the organism and are not given off to the 

 outside. Hence two groups of secretions can be distinguished, 

 according as after their formation they are at once given off or 

 are retained continually in the organism, whether in the cell or 

 upon its surface ; in neither case in the cell-community of the com- 

 pound organism is it always necessary that the secretion be of 

 use to that particular cell that affords it. 



Among the secretions that after their production learn the organism 

 there are, in the first place, the ferments, which have to do with 

 digestion and appear in both animals and plants. Thus, in 

 animals the cells of the salivary glands produce ptyalin, which 

 transforms starch into grape-sugar ; the cells of the gastric glands, 

 pepsin, which peptonises proteids, and rennet -ferment or chymosin, 

 which mediates the coagulation of casein ; and the cells of the 

 pancreas ptyalin for the digestion of starch, trypsin for the pep- 

 tonising of proteids, and steapsin for the splitting of fats. Ferments 

 occur likewise in plants, such as the so-called carnivorous plants, 

 which catch insects, hold them and digest them by the secretion of 

 peptonising ferments. An example of such a plant is Drosera, which 

 grows in the swamps. Whether the very effective ferments that are 

 produced in the milky juice of some plants, such as Carica papaya, 

 and are not cast out upon the surface, are to be regarded really as 

 secretions in the present sense or only as excretions (by-products of 

 metabolism) is thus far not decided, since the significance of these 

 in the life of the plant has not yet been discovered. In unicellular 

 organisms, further, the ferments are of great importance for the 

 nutrition of the cell when these organisms, as is the case with the 

 bacteria, come into contact with organic food and are obliged first 

 to liquefy solid food-stuffs in order to be able to absorb them. 



Other secretions, such as the wide-spread mucin, of which mucus 

 consists, are of great importance. Mucin protects the cell itself 

 from external influences that can harm it, such as direct contact 

 with objects; with strong stimulation the mucous cell produces 

 a thick layer of mucus separating the former from the body that 

 touches it ; this is the case with the mucous cells of the trachea 



