MICROSCOPIC FORMS OF VEGETABLE LIFE. 267 



265. Although it is probable that the Desmidiacew generally multiply 

 themselves also by the subdivision of their endochrome into a number 

 of zoospores, only one undoubted case of the kind has yet been recorded 

 (the Pediastrece, 270, being no longer ranked within this group); that, 

 namely, of Docidium Ehrenbergi, whose elongated cell' puts forth from 

 the vicinity of the sutural line, one, two, or three tubular extensions 

 resembling the finger of a glove, through which there pass out from 20 

 to 50 motile microgonidia formed by the breaking up of the endochrome 

 of the neighboring portion of each segment. 1 



266. Whether there is in this group anything that corresponds to the 

 ' encysting ' process ( 228 note) or the formation of ' stato-spores ' 

 ( 241) in other Protophytes, has not yet been certainly ascertained; but 

 the following observations may have reference to such a condition. It is 

 stated by Focke that the entire endochrome of Closterium sometimes 

 retracts itself from the cell-wall, and breaks itself up into a number of 

 globules, every one of which acquires a very firm envelope. And it is 

 affirmed by Mr. Jenner that " in all the Desmidiaceae, but especially in 

 Closterium and Micrasterias, small, compact, seed-like bodies of a 

 blackish color are at times to be met with. Their situation is uncertain, 

 and their number varies from one to four. In their immediate neigh- 

 borhood the endochrome is wanting, as if it had been required to form 

 them; but in the rest of the frond it retains its usual color and appear- 

 ance." It seems likely that, when thus inclosed in a firm cyst, the 

 gonidia are more capable of preserving their vitality, than they are when 

 destitute of such a protection; and that in this condition they may be 

 taken up and wafted through the air, so as to convey the species into new 

 localities. 



267. The proper Generative process in the Desmidiacem is always 

 accomplished by the act of ' conjugation;' which commences with the 

 dehiscence of the firm external envelope of each of the conjugating cells, 

 so as to separate it into two valves (Fig. 158, c, D; Fig. 159, c). The 

 contents of each cell thus set free without any distinct investment, blend 

 with those of the other; and a ' zygospore ' is formed by their union, 

 which soon acquires a truly membranous envelope. 2 This envelope is at 

 first very delicate, and is filled with green and granular contents; by 

 degrees the envelope acquires increased thickness, and it contents become 

 brown or red. The surface of the zygospore is sometimes smooth, as in 

 Closterium and its allies (Fig. 159); but in the Cosmariece, it becomes 

 granular, tuberculated, or even spinous (Fig. 158, D), the spines being 

 sometimes simple and sometimes forked at their extremities. 3 The mode 

 in which conjugation takes place in the filamentous species constituting 

 the Desmidiece proper, is, however, in many respects different. The 

 filaments first separate into their component joints; and when two cells 

 approach in conjugation, the outer cell- wall of each splits or gapes at 

 that part which adjoins the other cell, and a new growth takes place, 

 which forms a sort of connecting tube that unites the cavities of the two 

 cells (Fig. 160, D, E). Through this tube the entire endochrome of one 

 cell passes over into the cavity of the other (D) ; and the two are com- 



1 See Archer in " Quart. Journ. of Microsc. Sci.," Vol. viii. (1860), p. 227. 



2 In certain species of Closterium, as in many of the Diatomacece ( 280), the 

 act of conjugation gives origin to two sporangia. 



3 Bodies precisely resembling these, and almost certainly to be regarded as of 

 like kind, are often found fossilized in Flints, and have been described by Ehren- 

 berg as the remains of Animalcules, under the name of Xanthidia. 



