34 



PROTOPLASM 



cells may be greatly elongated, developing a branching, almost 

 treelike structure (Fig. 22). The cells of bone, connective tissue, 

 and cartilage secrete and embed themselves in a nonliving suVj- 

 stance just as the protoplast of the plant cell secretes its cellulose 

 wall. 



Unicellular Organisms. — We have so far had to do with cells 

 that are part of a larger living body. There are myriads of 

 different types of cells which live alone. Microscopic organisms 

 constitute a world of their own. Among the more lowly of one- 



FiG. 22. — Nerve 

 cell. {After L. W. 

 Sharp.) 



Fig. 23.— a yeast cell. {After 

 Smith.) 



celled organisms are those which cannot be definitely regarded 

 as either plant or animal. The bacteria are such cells. The 

 botanist speaks of bacterial flora, and the zoologist of a bacterial 

 fauna (flora is now mostly used). Similarities in the mode of 

 living of bacteria and fungi suggest the plantlike character of 

 the former. 



Bacteria are too small to permit a detailed study of their 

 structure. Some are barely visible with the highest power of 

 the microscope. They can, therefore, be classified only crudely 

 on the basis of their external form. Bacteria occur as spheres, 

 rods, and spirals. More satisfying and detailed is a physiological 

 and pathological classification, but this involves a knowledge of 

 medicine, hygiene, plant disease, fermentation, soil science, etc. 

 Slightly higher in the evolutionary scale is yeast (Fig. 23), 

 another unicellular plant. Euglena is a one-celled, border-line 



