Mr. J. Ralfs on the British Desmidiese. 375 



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

 M.ll.C.S., renzance*. 



In a former paper f I followed the example of most preceding 

 writers on these plants and referred them to the Diatomacecs; but 

 further observation has convinced me that they must be removed 

 from that tribe, which should comprise only the Algse with 

 siliceous covering, which I called Cymbellece. I have in that 

 paper pointed out many of the differences between these tribes ; 

 they also differ greatly in another respect. The Diatomacece 

 {Cymbellece) for the most part quickly acquire an offensive odour 

 after being gathered ; the Desmidiece, on the contrary, are remark- 

 able for the length of time they may be preserved in a moist state 

 without material change. Many indeed I have kept unaltered 

 for weeks in a damp piece of linen. As drying often produces a 

 great change in their appearance, it is fortunate that they can be 

 thus preserved until it is convenient to examine them. 



They are generally very minute, and, with the exception of a 

 few not hitherto detected in this country, are all found in fresh 

 waters, either mixed amongst other Algse or in old peat-pits, and 

 such shallow pools as do not become dry in summer J. 



These Algse have attracted but little attention from British 

 algologists, and only two species of Desmidium and two of Eu- 

 astrum are described in our Flora. I am convinced that these have 

 even a stronger claim to be considered plants than the Diato- 

 macece. This was also Meyen^s opinion ; for whilst he allows that 

 the true place of the latter is somewhat uncertain, yet, speaking 

 of the Desmidiea, he remarks, " This family includes those true 

 Algse of whose nature there can be no doubt." 



Ehrenberg, who refers them to the animal kingdom, lays the 

 greatest stress upon their " spontaneous division," which in- 

 deed is the only reason he produces for denying the vegetable 

 nature of some genera §. It has however been shown by Meyen, 

 Mr. Hassall || and others, that growth by the elongation and bi- 

 section of the cells is very frequent, if not universal, in the more 

 simple Algse. 



The Desmidiece have, in general, their cells more or less con- 

 stricted in the middle, and the endochrome divided into two por- 



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



t See Annals of Natural History, vol. xi. p. 448. 



X They frequently form finger-like tufts at the bottom of the pool, and if 

 gently separated by passing a knife or the finger beneath them, rise to the 

 surface, when they can be taken out and put into a bottle or placed on linen 

 and drained, and afterwards scraped oflT with a knife. 



§ " The increase by voluntary division is the character which separates 

 animals from plants." Ehr, See Annals of Nat. History, vol. ii, p. 123, 



II See Annals of Natural History, vol, ix. p. 431. 



