ON THE BRITISH SPECIES OE CRISTA. 129 



— representing the axis of a branch given oflF by the main stem 

 of a colony, together with all the ramifications of two of its 

 secondary branches. 



It will be noticed that nearly every internode develops a 

 single branch, and that the branches come off in regular 

 alternation on opposite sides of consecutive internodes of 

 every axis. Although the number of zooecia in the internode is 

 very variable, eleven may be regarded as the number most cha- 

 racteristic of the species. 



The above formula further shows that every branch-bearing 

 internode whose development is complete possesses an odd 

 number of zooecia, while in completely developed internodes 

 which bear no branch the number is even. Although this rule 

 is not quite absolute, it is difficult to find any exception to the 

 striking rule that a branchless internode has an even number 

 of zooecia ; or, conversely, that an internode with an even 

 number (whether this number is large or small) of zooecia 

 bears no branch. It may be pointed out that even if the 

 branches have been broken off, their previous presence can be 

 ascertained by the existence of the basal articulation from 

 which they formerly sprang. 



It may further be noted that a lateral branch is, with very 

 rare exceptions, produced on the side of the basal zocecium of 

 the internode (PI. XI, fig. 3). 



The regular alternation of the zooecia of the axis of any 

 branch is not disturbed by the development of an axial joint; 

 and the last zooecium of the internode below the joint nearly 

 always projects beyond the penultimate zooecium (which 

 belongs to the other side) in the form of a free tube (fig. 2). 

 Since the branch- bearing internode has an odd number of 

 zooecia, and since the branch is developed on the side of the 

 basal zooecium, it follows that the last zooeicum, which is 

 produced into a free tube, will also be on the same side as 

 the branch. A moment's consideration will show that the 

 basal zooecium, the branch, and the terminal ('' produced") 

 zooecium, in any internode, will normally be on the opposite 

 side to that on which these structures are situated, both in 



