CH. VIIl] 



OF SACHS'S EULES 



347 



for the area of this new partition is a x a/2. And again, for a 

 third bisection, our next partition must be perpendicular to the 

 other two, and it is obviously a little square, with an area of 



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

 body or parallelopiped to be divided equally by means of a 

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

 longest axis of the figure ; and (2) in the event of successive 

 bisections, each partition must run at right angles to its immediate 

 predecessor. 



a 



Fig. 136. (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 in full. 



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



(2) Each new plane of division tends to intersect at right 

 angles the preceding plane of division. 



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 hmiting our enquiry, for the most part, to cases of 

 equal subdivision. That it is by no means universally true for 

 cells generally is shewn, for instance, by such well-known cases 



