376 Mr. J. Ralfs on the British Desmidiese. 



tions. In Euastrum this constriction is so great that the fronds 

 seem to consist of two segments united by a narrow central chord, 

 whence most authors^ erroneously as I think, describe the plant 

 as binate. 



In the Diatomacecs, where the frustules are often truly binate, 

 as each frustule is complete in itself, though they be separated 

 from each other, their respective contents will still be protected 

 on all sides, and even if one be broken the contents of the other 

 will not be disturbed. In this tribe, on the other hand, as there 

 is no septum between the parts, if these separate or an opening 

 be made in one, the contents of both escape. In Desmidium the 

 constriction is often but slight, and although the endochrome is 

 most frequently in two portions, yet in an advanced state it is 

 sometimes collected into a single central spot. Whatever may be 

 the shape of the frond, this connecting portion is always nearly 

 or quite cylindrical ; and this is equally the case in the triangular 

 fronds of Staurastrum and the compressed ones of Euastrum, as in 

 those species having cylindrical fronds. Of course the more the 

 plant is compressed the narrower will be the connecting portion, 

 whilst in the cylindrical species the constriction is often but 

 slightly marked. In Closterium there is generally only a trans- 

 verse central line which divides the endochrome into two por- 

 tions ; but in all the Desmidiea, when the plant is mature, the 

 cells separate at the centre and allow the granules to escape. 

 In all the species, the growth by the repeated division of the 

 cells is extremely rapid. In Desmidium the process is exactly 

 similar to w^hat occurs in the Conjugates: the joint first elon- 

 gates, and then becomes double by the formation at the centre 

 of internal transverse septa ; but in most of the other DesmidiecB 

 the fronds are simple, or consist of only a single cell, which, as I 

 have observed above, is more or less evidently in two segments. 

 Euastrum has these united by a narrow chord, and therefore in 

 that genus the manner of their increase by division can be most 

 easily observed. The central chord elongates, and two new seg- 

 ments are formed, which gradually increase until they attain the 

 same size as the halves of the original frond. About this time it 

 separates into two distinct fronds, each of the old segments ha- 

 ving united with one of the new ones : during this process the 

 original halves do not undergo any alteration, except in being 

 separated by the two new segments, all the growth taking place 

 in the central chord that united them. As this addition is con- 

 tinually taking place in those fronds which have reached their full 

 size, the two segments of a frond are very often unequal. 



All the species are binate during the production of the new 

 portions and until separation takes place. 



In describing Meloseira, Isthmia, &c., I have shown that those 



