VIIl] 



OF THE GROWING POINT 



637 



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

 is as much as to say that the more actively growing pro'toplasm, 

 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 



4 5 6 7 



Fig. 282. Development of antheridium of liverwort (diagrammatic). 



the act of division be so long postponed that the cell has become 

 quasi-cylindrical) it will divide by another obhque 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 a single 

 apical cell, of a triangular form. The developing antheridium of a 

 liverwort (Riccia) is a typical example of such a case. In Fig. 283 

 which represents a "gemma" of a moss, we see just the same thing; 

 with this addition, that here the lower of the two original cells has 

 grown even more quickly than the other, constituting a long cyhn- 



* Cf. p. 590. 



