VIII] OR CELL-AGGREGATES 403 



the growth of the whole system is mainly in a vertical direction, 

 which is as much as to say that the more actively growing proto- 

 plasm, or at least the strongest osmotic force, will be found 

 near the apex; where indeed there is obviously more external 

 surface for osmotic action. It will therefore be that one of 

 the two cells which contains, or constitutes, the apex which 

 will grow more rapidly than the other, and which therefore will 

 be the first to divide, and indeed in any case, it will usually be 

 this one of the two which will tend to divide first, inasmuch 

 as the triangular and not the quadrangular half is bound to 

 constitute the apex*. It is obvious that (unless the act of division 

 be so long postponed that the cell has become quasi-cylindrical) 

 it will divide by another oblique partition, starting from, and 

 running at right angles to, the first. And so division will proceed, 

 by oblique alternate partitions, each one tending to 

 be, at first, perpendicular to that on which it is based 

 and also to the peripheral wall ; but all these points of 

 contact soon tending, by reason of the equal tensions 

 of the three films or surfaces which meet there, to form 

 angles of 120°. There will always be, in such a case, 

 a single apical cell, of a more or less distinctly 

 triangular form. The annexed figure of the developing 

 antheridium of a Liverwort (Riccia) is a typical example 

 of such a case. In Fia;. 185 which represents a 



. . Fio-. 185. 



"gemma" of a Moss, we see just the same thing; Gemma of 

 with this addition, that here the lower of the two ^^^- {^^^^^^ 



Campbell.) 

 original cells has grown even more quickly than the 



other, constituting a long cylindrical stalk, and dividing in ac- 

 cordance with its shape, by means of transverse septa. 



In all such cases as these, the cells whose development we have 

 studied will in turn tend to subdivide, and the manner in which 

 they will do so must depend upon their own proportions ; and in 

 all cases, as we have already seen, there will sooner or later be 

 a tendency to the formation of periclinal walls, cutting off an 

 "epidermal layer of cells," as Fig. 186 illustrates very well. 



The method of division by means of oblique partitions is a 

 common one in the case of ' growing points ' ; for it evidently 



* Cf. p. 369. 



26—2 



