PHYSIOLOGY OF CELLS 59 



through the cell membranes to the exterior, or in some of the Protozoa 

 they are gathered up by the contractile vacuoles and voided through 

 the outlets of these organs. 



Plants in the course of their metabolic activities produce a number 

 of substances, such as tannin and probably oxalic acid, which are often 

 of no further use to the plant cells. Such substances may then be con- 

 sidered excretions which, however, the plants have insufficient means of 

 eliminating. In some cases the cells render them inert by changing them 

 from solutions to a crystalline form. Thus oxalic acid is neutralized 

 by calcium salts in the cells with the formation of insoluble cal(;ium 

 oxalate. Salts are known to be exuded upon the surfaces of plants, 

 where they are washed away by rain or dew, but it is not certain that 

 these substances are to be regarded as excretions. 



Secretion. — The cell is to be regarded as a laboratory or factory in 

 which a multitude of chemical processes are taking place. Some of these 

 processes have already been discussed. All cells produce certain chemical 

 compounds which may be used in the chemical processes going on within 

 the cell or in cavities adjoining the cells. Such products are secretions 

 and the process of producing them is given the name secretion. Secre- 

 tions differ from excretions in that they are used in performing some 

 function for the body, while excretions cannot ordinarily be used by the 

 organism. The actual processes involved in the production of excretions 

 and secretions are very similar and the methods by which these substances 

 are discharged from the cells of multicellular animals may be identical. 

 Many of the secretions which are discharged from the cells are first stored 

 in the cells as granules which finally break out of the cell at the exposed 

 end and then become liquid or gaseous. Other secretions produced as 

 liquids within the cell diffuse out and escape as rapidly as formed, are 

 absorbed by other cells, or are carried in the blood stream. Such secre- 

 tions may perform their functions at a considerable distance from the 

 cells where they are elaborated. Secretions are very diverse in their 

 functions. Some aid in digestion, others give protection because of their 

 odor or because of poisonous qualities, some serve as lubricating mate- 

 rial, others oxidize readily with the production of light, and there are 

 still other kinds performing other functions. The diversity of functions 

 served by secretions is indeed very great. 



Single-celled animals and plants produce all the secretions required 

 for carrying on chemical processes within their single cells. In higher 

 animals and plants specialized cells may be set aside chiefly for the 

 production of certain secretions. Localized groups of these cells are 

 called glands. The structure of glands is considered in Chapter VI. 



Growth. — Growth is characteristic of living organisms. It is due to 

 the conversion of foods into protoplasm at a more rapid rate than pro- 

 toplasm is bpijig broken down in catabolic processes. Cells are strictly, 



