Minnesota Plant Life. 493 



certain oceanic relatives of the green felt an alga belonging 

 to the bright-green group. Yet even in the latter, the body, 

 as large as a hen's egg, which exists without partition walls 

 cutting it in all the planes of space is equivalent rather to a 

 cell-aggregate than to a single cell. 



Just why the limits between which cells vary in size should 

 be what they are, is a difficult question to answer. The cause 

 must be sought in the structure and qualities of the living sub- 

 stance and in the conditions of the outer world as they react 

 upon it. A proper comparison would be with the towns and 

 cities of the human species. These vary in size between mere 

 hamlets to cities like London, Paris and New York. Cities 

 with fifty million inhabitants, however, do not exist, because 

 with human society organized as it is they cannot be main- 

 tained. To explain exactly the causes that have fixed 6,000,- 

 ooo rather than some other number as a population limit be- 

 yond which human cities have not yet developed, would re- 

 quire a more exhaustive knowledge of anthropology and soci- 

 ology than any one possesses. Still more impossible is it to 

 state why the limits of cell size are precisely what they are. It 

 is, at the same time, clear that, as in the instance of the cities, 

 the reason lies essentially in the nature of the organism that 

 is, the living substance which builds the cell. 



The largest masses of living substance in which cell com- 

 partments have not appeared are the jelly-like bodies of the 

 slime-moulds. These aggregates of protoplasm, in patches 

 the size of a dinner plate, are sometimes found on decaying 

 timber. Because they are not provided with internal mechan- 

 ical support they lie flat and shapeless upon their substratum. 



Origin of the cell as a structural unit. One is now in a 

 position to understand why living substance almost universally 

 displays itself in the microscopic individualized portions known 

 as cells. The cell is an adaptation fitted to enable the living 

 substance, under the stress of outward conditions, to perform 

 its physiological functions in a better way than if this adapta- 

 tion had not been called into being. There is reason to sup- 

 pose that mechanical support and protection against shock 

 might have been the principal necessities under which primal 

 masses of living substance came to develop the cell-habit. In 



