196 NUTRITION AND METABOLISM 



compounds enter as are permitted by the cell-wall and protoplasm by 

 means of osmotic pressure. They then diffuse throughout the proto- 

 plasm of the cell. Other digestive agents within the cell make the food 

 assimilable. In molds the food may apparently pass along the myce- 

 lium or hyprue, in other words be transmitted for some distance through 

 the organism. In the case of the yeast cell and bacteria the process is 

 very similar but the transmission of nutritive material beyond a single 

 cell is not known to take place and perhaps there is no need for it. 

 Whether food is conveyed from one cell to another in colonies has not 

 been" determined so far as the writer knows. 



Food 



Waste 

 Secretions 



Water 



FIG. no. Illustrating cell activities. 



Waste products resulting from the metabolism of protoplasm leave 

 the cell through the cell- wall, also by means of osmosis, and this process 

 appears to be the same for the ingestion of food as for the egestion of 

 waste products. 



Some microorganisms live upon dead matter, some upon living 

 matter and some may make use of either. The greater portion, by far, 

 require or prefer organic substances. When organisms, as protozoa, 

 feed upon living organisms they are said to be holozoic in their mode of 

 life, in other words they follow closely the methods employed by ani- 

 mals. Then there are those protozoal organisms which simulate plants 

 in their manner of nourishment. These are called holophytic. This 

 latter class is associated with the formation of chlorophyll-bodies within 

 their structure. There are those organisms, too, which consume 

 organic matter which is rendered suitable by nature or decay, called 

 saprozoic or saprophytic, depending upon whether the organism is 

 designated as animal or plant. Whenever organisms require living 

 tissues to sustain life, in the form of a host, they are called parasitic. 



