VIl] 



OF SACHS'S RULE 



475 



and is at right angles to the axis of symmetry. The actual curvature 

 of the partition- wall is easily seen in optical section ; but in surface 

 view the line of junction is projected as a plane (Fig. 160), perpen- 

 dicular to the axis, and this appearance has helped to lend support 

 and authority to "Sachs's Rule." 



Fig. 159. Fig. 160. 



As soon as the tensions of the cell- walls become unequal, whether 

 from changes in their own substance or in the substances with 

 which they are in contact, then the form alters. If the tension 

 along the partition P diminishes, the partition itself enlarges and 

 the angle QPR increases: until, when the tension p is very small 

 compared with q or r, the whole figure becomes a sphere, and the 

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

 angles to the outer wall. This is the case w^hen the outer wall of 

 the cell is practically sohd. On the other hand, if p begins to 



Fig. 161. 

 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 tw^o 

 separate cells. 



To put the matter still more simply, let the annexed diagrams 

 (Fig. 161) represent a system of three films, one being a partition- 



