ON THE BBITISH SPECIES OF OBISIA. 139 



replace the third zooecium above a joint (and it is then abaxial), 

 but it is very rarely found higher in the internode. A branch 

 is never given off by an ovicell. 



As the age of the ovicell increases, fresh zooecia continue to 

 be added above it up to a certain point. The old ovicell seems 

 to be always surmounted by a considerable number of zooecia ; 

 in the specimen shown in fig. 6 there are, in addition to two 

 incompletely formed zooecia — the last that this branch would 

 have produced — ten zooecia above the ovicell. It must be noted 

 that in this and other similar cases all zooecia which are 

 further from the joint than the ovicell are described as being 

 above the latter. The second zooecium of the right side in 

 fig. 6 may not, at first sight, appear to be in this position, 

 although an examination of the lower end of the ovicell at once 

 shows its real place in the series. 



A joint is seldom developed above the ovicell, and the growing- 

 point usually completely exhausts its power of developing fresh 

 zooecia after a certain period. 



The j oints of this species are pale-coloured, or more usually 

 yellow. In old parts of the colony the joints may become very 

 dark, or almost black ; this is especially true of those parts 

 which form the starting-point for the regeneration of fresh 

 branches. The joints are probably never so dark as they are 

 normally in C. denticulata. 



Smitt, in his valuable paper on Crisia,^ gives a series of 

 formulae illustrative of the branching, &c., of the forms of this 

 genus, and many of these formulse illustrate in a most instructive 

 manner the tendency of some at least of the species of Crisia 

 to develop even-numbered internodes without branches. In his 

 explanation to No. 8 of this series Smitt expressly points out 

 that, in Nos. 4 — 8, shorter branchless internodes may alternate 

 with longer internodes which have developed branches. It is a 

 noteworthy fact that the greater number of the branchless 

 internodes shown in these formulae have an even number of 

 zooecia, and that the number is odd in most of those internodes 

 which have developed branches. This fact seems, however, to 

 I « Krit. Forteokn.," I, ' Ofvers. af K. Vet.-Akad. Porhandl.,' 1865. 



