ON THE BTIITTSH SPECIES OE CEISIA. 133 



overlapj and be fused with the next zooecium higher up on the 

 same side if it were not for the development of the joint ; which 

 is, however, formed in such a position across a zooecium as to 

 leave the preceding zooecium in the characteristic " produced " 

 condition which has already been described. In the ordinary 

 type of branching, where successive internodes produce branches 

 in regular alternation on opposite sides, the number of zooecia 

 must be odd if the branch is to be produced on the side of the 

 basal zooecium in each internode. The formation of a new 

 axial internode practically amounts to the transverse division 

 of a zooecium, while the formation of a branch may be ex- 

 pressed as due to the longitudinal division of a zooecium (at 

 the growing-point). Suppose that the right side of an axis 

 bears a branch (as in the lowest internode shown in fig. 2). 

 The tendency of the growing-point to produce new branches 

 alternately on opposite sides would normally result in the pro- 

 duction of a branch from the left side of the next youngest 

 internode ; but if a lateral branch has not been produced 

 by the time that a new axial joint is to be formed, that axial 

 joint would be, as a matter of fact, normally developed from a 

 zooecium of the left side, as at the base of the third internode 

 in fig. 2; and this implies the existence of an even number 

 of zooecia in the second internode. The production of an even- 

 numbered internode may thus be regarded as due to the alter- 

 nate predominance of the two sides of the growing-point. The 

 development of a lateral branch on the right side (e. g.) has 

 apparently the effect of leaving the left side of the growing-point 

 with an excess of vigour, so that when a new internode is 

 formed — whether by the transverse division of a zooecium to 

 form an axial joint, or by its longitudinal division to form a 

 lateral branch — it is the left side (in this particular case) of the 

 growing-point by which this division is efi'ected. Division in 

 the transverse direction results in the formation of an even- 

 numbered internode, while the production of a lateral branch 

 on the left side of the next succeeding internode restores the 

 function of producing another axial joint to the right side of 

 the growing-point. 



