4 PROTOPLASM AND PLANT CELLS 



theories as to its physical structure that receive the 

 strongest support are the emulsion and the fibrillar 

 theories respectively. By the first theory protoplasm 

 is a very complex emulsion of various substances more 

 or less closely related chemically. The bodies appear- 

 ing as granules would be then, in part at least, small 

 drops suspended in the emulsion. These drops are 

 perhaps themselves also emulsions. The fine lines visi- 

 ble under certain conditions would be not fine strands 

 but rather the edges of surfaces separating adjacent 

 units of the emulsion. It is readily seen that this theory 

 would accord well with the observed fact of the great 

 power of imbibition of water by the protoplasm, for this 

 would but separate the droplets of the emulsion some- 

 what more without necessarily disturbing their relative 

 positions. The viscidity or relative firmness of some pro- 

 toplasm (e.g. plastids and nucleus) is in agreement with 

 what we know about emulsions. Thus two thin liquids 

 may sometimes be brought to such a state of emulsion 

 that the whole mass is firm and will stand upright. The 

 fibrillar theory supposes that the delicate lines mentioned 

 above are fine threads, connected at innumerable points 

 and traversing the clear liquid making up the bulk of the 

 protoplasm. The granules are looked upon as being 

 situated on these fibrillac or sometimes in the spaces 

 between them. 



8. The Plant Cell. In all plants we find that the 

 protoplasm occurs in definite units which are independ- 

 ent or more or less connected with neighboring units; in 

 the latter case the whole mass of these units constitutes 

 the plant. These units are called cells and consist 

 always of at least two parts, a mass of cytoplasm and a 

 nucleus. In most plant cells the protoplasm deposits a 

 firmer substance as a box-like covering called the cell wall, 



