302 THE FORMS OF TISSUES [ch. 



along the partition, P, diminishes, the partition itself enlarges, 

 and the angle QOR increases : until, when the tension P is very 

 small compared to Q or R, the whole figure becomes a circle, and 

 the partition-wall, dividing it into two hemispheres, stands at 

 right angles to the outer wall. This is the case when the outer 

 wall of the cell is practically solid. On the other hand, if P begins 

 to increase relatively to Q and R, then the partition- wall contracts, 

 and the two adjacent cells become larger and larger segments of 

 a sphere, until at length the system becomes divided into two 

 separate cells. 



In the spores of Liverworts (such as Pellia), the first partition- 

 wall (the equatorial partition in Fig. 109, 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 



Fig. 109. Spore of Pellia. (After Campbell.) 



end of the original spherical or elliptical cell. Each of these latter 

 partitions \vill (like the first) tend to set itself normally to the 

 cell-wall; at least the angles on either side of the partition w^ll 

 be identical, and their magnitude will depend upon the tension 

 existing between the cell-wall and the surrounding medium. 

 They will only be right angles if the cell- wall is already practically 

 solid, and in all probability (rigidity of the cell- wall not being 

 quite attained) they will be somewhat greater. In either case 

 the partition itself will be a portion of a sphere, whose curvature 

 will now denote a difference of pressures in the two chambers or 

 cells, which it serves to separate. (The later stages of cell-division, 

 represented in the figures h and c, we are not yet in a position to 

 deal with.) 



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

 where in like manner the partition- wall which cuts ofE the terminal 



