144 VEGETABLE CELLS. 



this is by no means the rule. When the cell does become 

 altered in shape, it much more frequently happens that 

 it passes from the polyhedral into the spherical, then 

 from the spherical into the polyhedral form. This last, 

 indeed, I believe only occurs in the endosperm-cells. 



The growth of the cells is of two kinds. Either the 

 whole contents of the cell are simultaneously transformed, 

 and the entire membrane expands at once I will call this 

 universal growth (allseitiges Wachsthum}, or new contents 

 are continuously formed at one point of the surface of the 

 cell, and with this new membrane I call this apical 

 growth (Spitzenwachsthum). 



In the growth of the cell it is necessary to distinguish 

 between the growth of the contents and that of the mem- 

 brane. The former appears always as causal and primary, 

 the latter as caused and secondary. I shall here speak 

 only of the growth of the membrane. This is a growth 

 in thickness, and a growth in surface. The latter comes 

 particularly into consideration in treating of the growth 

 of the cell. It is conceived as an expansion resulting 

 from the intussusception of organic molecules, and called 

 a nutrition of the membrane. Without entering into any 

 criticism of this theory, I will merely investigate, with 

 what phenomena the growth of the membrane in surface 

 is connected, in universal and apical growth of the cell. 



When, in universal growth, the cell does not change 

 its form, the membrane must expand uniformly in all 

 parts. On the other hand, when the shape is altered, 

 some parts of the membrane must expand more than 

 others. The membrane consists of surfaces. In each of 

 these faces the growth of the membrane must be decom- 

 posed into two factors, the expansion in the two dimensions 

 of a plane. In each dimension we have three possibilities : 

 either the expansion is +#?, 0, or x\ either there is an 

 expansion or none ; or a diminution of the face of the 

 membrane in this dimension. The last case, though rare, 

 does occur in a few instances. The growth of the surface 

 of a membrane, therefore, when the three possibilities in 



