10 PLANT PHYSIOLOGY 



that are connected by means of oxygen bridges. Each molecule 

 of cellulose contains about 40 such cycles stretched in a chain, 

 and as their number is not constant, it is natural that in applica- 

 tion to cellulose and to similar polymolecular substances, the 

 concept of the size of the molecule differs somewhat from its 

 usual significance; thereby, the difference between molecules 

 and micelles disappears. The length of such a molecule of 



o 



cellulose approximates 200 angstrom units (A.), and its molecular 

 weight approaches 30,000 to 40,000. 



The cellulose molecules arranged in chainlike form are com- 

 bined in their turn into groups holding about 60 chains each and 

 displaying a crystalline structure. The cell wall consists of 

 such aggregates, arranged somewhat like bricks forming a wall. 



In elongated fibrous cells, e.g., in cotton or flax fibers, all the 

 molecular groups are stretched in one direction, forming very 

 fine fibrils. In cells of a parenchymatous character, the groups 

 are formed with less regularity. Between these aggregates, there 

 are interstices producing a finely porous structure of the cell 

 wall. 



The structure of the protoplasm is of a completely different 

 character. Owing to the firm connection between the micelles, 

 the cell wall has the character of a solid body and is capable of 

 forming the particular shape of the cell, which is frequently of 

 specialized type. Protoplasm itself is commonly a rather viscous 

 fluid. When hberated from the cell wall, for instance, during 

 the formation of zoospores in fungi and algae, it assumes a 

 spherical shape. The streaming movements of protoplasm in 

 cells that may be frequently observed demonstrate its liquid 

 nature. 



Although protoplasm is mainly fluid in nature, it is by no 

 means a uniform liquid. It is an emulsion, the dispersed phase 

 being composed of fats and lipoids, while the dispersion medium 

 is a hydrosol containing a large amount of protein and other 

 substances, as well as simple and complex ions. I'he substances 

 forming the protoplasm are in a state of continuous interaction. 

 Frequently they display partial coagulation and gelatinization, 

 as well as the formation of a fibrillous structure somewhat similar 

 to the structure of cellulose. The fil^rils form a fine network 

 interspersing the basic hquid mass of the protoplasm. The 

 presence of such a fine network of ultramicroscopic fibrils gives 



