GROWTH OF CELLS. 145 



each dimension are combined, exhibits, in general, nine 

 possible cases, leaving out of the question that a multitude 

 of differences in quantity may occur. In a cell where 

 the growth is not uniform, we must distinguish at least 

 between two faces ; but cells occur with ten, twenty, and 

 thirty faces. Prom this it may be conceived what an infinity 

 of theoretically possible diversities exist for the universal 

 growth of a cell-mernbrane in its totality, since this pro- 

 ceeds from the combination of so many and, more than 

 this, such variable factors. I think, therefore, that in 

 particular cases the growth of the membrane may be 

 decomposed into its factors, but that it is impossible to 

 establish general rules. 



Schleiden* considers that the variations of form of 

 cells result from unequal nutrition of the membrane, this 

 taking place only in those places where one cell is in con- 

 tact with another, or with a fluid ; moreover is more 

 vigorous in those situations where a more considerable 

 interchange of matters with other cells is going on, thus 

 at the ends of elongated cells more strongly than at their 

 lateral faces, f 



Schleiden here starts from the facts, that the cells of 

 the epidermis and of the septa in the air-canals are 

 flattened ; that in stellate and spongiform cells only the 

 rays are in contact with other cells ; that where a current 

 of sap passes through a tissue, elongated cells are formed. 

 The example cited, of Caulerpa, does not belong here, 

 but to a growth of quite different kind, to apical growth. 



The facts brought forward show the incontestible rule, 

 if not absolute and unexceptional law ; but I draw quite 

 a different conclusion from them, namely, that the cell- 

 membrane grows least exactly where the greatest inter- 

 change of substances with external bodies takes place, and 

 that it expands most where it is least occupied in receiv- 

 ing and giving off substances. 



I will first mention some negative reasons against 



* Grundziige d. wis. Bot. } 3d ed. i, p. 211. 

 f L. c. 250. 



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