478 



THE FORMS OF TISSUES 



[CH. 



(the equatorial partition in Fig. 165 a) divides the spore into two 

 equal halves, and is therefore a plane surface normal to the surface 

 of the cell. But the next partitions arise near to either end of the 

 original spherical or elliptical cell, and each of these latter will 

 likewise tend to set itself normally to the cell-wall — at least the 



Fig. 164. 



angles on either side of the partition will tend to be identical, and 

 their magnitude will depend on the relative tensions of the cell- wall 

 and the partition. The angles will be right angles if the cell-wall is 

 solid or nearly so when the partition is formed; but they will be 

 somewhat greater, if (in all probability) rigidity of the cell-wall 

 has not been quite attained. In either case the partition itself will 



Fig. 165. Early development of a liverwort (Pellia). After Wildeman. 



be part of a spherical surface, whose curvature will now correspond 

 to the difference of pressures in the two chambers (or cells) which 

 it serves to separate. 



We have innumerable cases, near the tip of a growing filament 

 for instance, where in like manner the partition-wall which cuts off 

 the terminal, more or less conical, cell constitutes a spherical lens- 



