CH.viii] OF SACHS'S RULES 567 



From this we may draw the simple rule that, for a rectangular 

 body or parallelepiped to be bisected by means of a partition of 

 minimal area, (1) the partition must cut across the longest axis of 

 the figure; and (2) in successive bisections, each partition must run. 

 at right angles to its immediate predecessor. 



X 



Fig. 219. After Berthold. 



We have already spoken of "Sachs's Rules," which are an 

 empirical statement of the method of cell-division in plant-tissues ; 

 and we may now set them forth as follows : 



(1) The cell tends to divide into two co-equal parts. 



(2) Each new plane of division tends to intersect thCj preceding 

 plane of division at right angles. 



The first of these rules is a statement of physiological fact, not 

 without its exceptions, but so generally true that it will justify us 

 in limiting our enquiry for the most part to cases of equal sub- 

 division That it is by no means universally true for cells generally 

 is shewn, for instance, by such well-known cases as the unequal 

 segmentation of the frog's egg. It is true, when the dividing cell 

 is homogeneous and under the influence of symmetrical forces ; but 

 it ceases to be true when the field is no longer dynamically sym- 

 metrical, as when the parts differ in surface tension or internal 

 pressure, or, speaking generally, in their chemico-physical properties 



