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 fibrillae 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, 
