CHAPTER I. — THE CELL. I 25 



though in some cases they are large enough to be distinctly seen 

 by the unaided eye, as in the flesh of the Water-melon and the 

 pith of the Elder ; in rare instances, as in the inter-nodal cells 

 of Chara and Nitella, they may even be upwards of an inch in 

 length. Some, on the other hand, are so small as to be barely 

 visible under the highest powers of the microscope. This is the 

 case with the cells of some Bacteria, and there is good reason to 

 believe that there are some low organisms belonging to this type 

 which no microscope yet made is powerful enough to resolve. 



The primary form of cells appears to be that of a sphere, or 

 a spheroid, but commonly, especially as we find them in the 

 tissues of the higher plants, they acquire forms quite different 

 from this, and, even within the limits of the same organism, the 

 shapes may be exceedingly various. This may be due to mutual 

 pressure, to unequal growth caused by the unequal operation of 

 various physical forces, as gravitation, light, etc., or to other 

 influences. Cells, like the organs of which they are com- 

 ponents, undergo many modifications of form and structure, 

 adapting them to different uses. The cells which make up the 

 body of a living tree are comparable to the human units which 

 make up society. As in the latter case, the various individuals 

 resemble each other fundamentally, — that is, as to origin, general 

 endowments, and essential structure, — so do they also in the 

 former. As in the social body men differ in rank, in occupation, 

 in mode of life, and in habits of thought, thus producing differ- 

 ent classes, each contributing to the structural whole, so also 

 cells are divided into classes, each class having a somewhat 

 different function, but each contributing to the life of the whole 

 organism ; and as in society individuals come and go, while the 

 community as a whole lives on, so in the tree cells come into 

 existence, live their short lives, perish, and are succeeded by 

 others, while the life of the organism, as a whole, continues. A 

 plant, then, at least one of the higher plants, is a community or 

 republic of cells. To understand it, one must understand the 

 individuals that compose it. Let us first, therefore, study the 

 cell in its typical form, and then afterward in its various modifi- 

 cations. 



The typical vegetable cell is a spherical or spheroidal body 

 like that represented in Fig. 361. It has an outer membranous 



