Mr. J. Ralfs on the British Desmidieae. 187 



to which it may natm-ally be placed. It differs in being much 

 more branched, in its spiry habit and deHcate substance, and, 

 more definitely, in having the opposite ramuli very much shorter 

 and invariably simple and subulate. In the other British species 

 with opposite simple ramuli (C Turneri, Plumu and harbatum) the 

 ramuli do not issue from every joint of the frond, nor do they 

 spring from a point below that of the diaphragm. These cha- 

 racters are pecidiar to C. Plumula (a species so different from 

 that under consideration that I need not compare it), to C. cru- 

 ciatum and C. Pollexfenii. And in another remarkable pecu- 

 liarity these latter species also agree, namely, that the ramifica- 

 tion proceeds on a plan different fi'om that of the ramulifi cation. 

 In most Algae with decompoimd fronds, the normal division of the 

 branches is like\vise that of the ramuli. In these it is the reverse, 

 the branches being invariably alternate or scattered, and the ra- 

 muli as invariably opposite. Here too we never find the ramuli 

 lengthening into branches, and they are nearly of the same size 

 on every part of the frond. And so constantly are they produced 

 by every articulation, that even when a branch is given off, the 

 ramuli of the joint from whose apex it springs are as fully de- 

 veloped as those of any simple joint. Tlie ramidi in these spe- 

 cies have therefore more affinity with true leaves than with young 

 branches. W. H. H. 



July 16, 1844. 



EXPLANATION OF PLATE V. 



Fig. 5, Cal. Pollexfenii, nat. size. 



Fig. 6. Portion of a branch, magnified. 



Fig. 7. Joints of the stem, to show the insertion of the ramuli, magnified. 



XXV. — On the British Desmidiese. By John Ralfs, Esq., 

 M.R.C.S., Penzance*. 



[With a Plate.] 

 EuASTRUM, Ehr. 

 Frond simple, compressed, deeply divided into two segments 

 which are emarginate at their ends, lobed or sinuated and ge- 

 nerally pjTamidal. 



The fronds are simple, longer than broad, often oblong, com- 

 pressed, and so deeply constricted that their segments seem only 

 united by a narrow chord. The generally pyi-amidal segments 

 are broadest at their bases, and are there in such close apposition 

 for their entire breadth as nearly to conceal the notch on each 

 side until the endochrome has collapsed. They are attenuated 

 towards the ends, which in the adult state are always more or less 



* Read before the Botanical Society of Edinburgh, April 11, 1844. 



02 



