GROWTH OF CELLS.

145

*

each dimension are combined, exhibits, in general, ninepossible cases, leaving out of the question that a multitudeof differences in quantity may occur. In a cell wherethe growth is not uniform, we must distinguish at leastbetween two faces; but cells occur with ten, twenty, andthirty faces. From this it may be conceived what an infinityof theoretically possible diversities exist for the universalgrowth of a cell-membrane in its totality, since this pro-ceeds from the combination of so many and, more thanthis, such variable factors. I think, therefore, that inparticular cases the growth of the membrane may bedecomposed into its factors, but that it is impossible toestablish general rules.

Schleiden* considers that the variations of form ofcells result from unequal nutrition of the membrane, thistaking place only in those places where one cell is in con-tact with another, or with a fluid;—moreover is morevigorous in those situations where a more considerableinterchange of matters with other cells is going on, thusat the ends of elongated cells more strongly than at theirlateral faces, f

Schleiden here starts from the facts, that the cells ofthe epidermis and of the septa in the air-canals areflattened; that in stellate and spongiform cells only therays are in contact with other cells; that where a currentof 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 quitea different conclusion from them, namely, that the cell-membrane grows least exactly where the greatest inter-change of substances with external bodies takes place, andthat 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. -wiss. Bot., 2d ed. i, p. 211.t L. c. 250.

10