34^ LECTURE XV. 



4. Influence of the Tensions upon the Growth of the 



Cells. 



We gather from what has already been said that the wall 

 of a growing cell is subjected in all cases to a pressure from 

 within, due to its own turgidity, and, in the case of a cell 

 forming part of a tissue, to a pressure from without exerted 

 upon it by the surrounding cells. It has been already men- 

 tioned that each cell tends in the course of its growth to 

 assume a particular form, "and we have now to determine in 

 the first place the mode in which this tendency obtains ex- 

 pression, and secondly, the effect of the pressure of surround- 

 ing cells in modifying the form which the cell naturally tends 

 to assume. 



In discussing the former of these points, let us first con- 

 sider the case of a cell, a unicellular plant for example, which 

 is free, that is, not coherent with other cells. In this case 

 only the internal pressure due to turgidity has to be taken 

 into account. The form assumed under such circumstances 

 by the cell in the course of its growth is determined by the 

 distribution of extensibility in the cell-wall. If the cell-wall 

 be uniformly extensible, then, since the hydrostatic pressure 

 is the same at all points of its internal surface, the cell-wall 

 will be uniformly stretched, and the general form of the cell 

 will continue to be that which it originally possessed. If, 

 however, the cell-wall be not uniformly extensible, but some 

 areas are more extensible than others, then the more exten- 

 sible areas will yield to the internal pressure, and the result 

 will be the formation of outgrowths and a complete change in 

 the form of the cell. 



The inequalities of extensibility in the wall of a growing 

 cell can only be ascribed to differences of structure in different 

 parts. These differences of structure may depend upon 

 differences in thickness due to an unequal deposition of 

 cellulose on the internal surface of the wall, or upon an 

 alteration in the molecular structure of the wall effected 

 by the protoplasm, the result of which is to cause 



