52 OF THE DESMIDIACE.E AND SIMILAR MINUTE ALGM. 



In some instances, the individual is constricted in such a 

 manner, as to have the appearance of two valves united 

 together centrally : of these, fig. 58, Plate xn., is an example. 



The reader will observe the same peculiarity of structure 

 in the figures ranging from 17 to 22, already given in Plates 

 iv. v. Other species, again, are arranged in long filaments ; 

 or ribbon-like colonies, fig. 59, Plate xiii. 



These filamentous Desmids generally yield very clean 

 preparations. 



It will be seen that Hyalotheca is enveloped in a thick 

 coating of mucous matter : this is by no means a common 

 form of construction, though we find instances here "and 

 there, of which Spirotsenia condensata, Plate xiii., fig 60, is 

 an example. 



The decayed remains of Sphagnum in peat bogs are 

 often rich with the empty frustules, well adapted for ex- 

 amination, though, from their dirty brown colour, and from 

 being mingled with the surrounding vegetable matter, they 

 are not fit specimens for the herbarium. Having lost the 

 bright green of the living Desmid, they are difficult to dis- 

 tinguish with the naked eye ; in fact, they are mere empty 

 cases ; but none the less welcome to the collector on that 

 account, as they often give him opportunities of studying 

 the cell membrane, which he cannot get with specimens 

 better adapted for preservation. And here let me recom- 

 mend him to be always provided with plenty of phials on 

 his excursions, that each gathering may be kept apart ; 

 otherwise he entails upon himself a vast amount of trouble, 

 when he comes to separate the species for preparation. 



Many of the DesmidiaceaB will readily propagate when 

 kept in a room in a sufficient quantity of bog-water, thus 

 enabling one to get an abundant supply of clean specimens. 

 The plate, or saucer, in which they are placed, should be 

 covered with a sheet of glass to keep out the dust, and to 

 prevent the water from evaporating too rapidly. However, 

 should the water get too low from this or any other causo, 

 the supply must be made up with rain or soft water, on no 



