274 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



Fio. 75 



but especially in Closterium and Mierasterias, 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 neighborhood the endochrome is want- 

 ing, as if it had been required to form them ; hut in the rest of 

 the frond it retains its usual color and appearance." It seems 

 likely that, when thus enclosed 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. 



169. The proper Generative process in the Desmidiacem is 

 always accomplished by the act of " conjugation ;" and this takes 

 place after a manner very difterent from that which we have 



seen to occur in Palmoglsea. For 

 each cell here possesses, it will be 

 recollected, a firm external enve- 

 lope, which cannot enter into coa- 

 lescence with that of any other; 

 and this membrane dehisces more 

 or less completely, so as to sepa- 

 rate each of the conjugating cells 

 into two valves (Fig. 75, c, d ; Fig. 

 76, c). The contents of each cell, 

 being thus set free, without (as it 

 appears) any distinct investment, 

 blend with those of the other ; and 

 a mass is formed by their union, 

 which so acquires a truly mem- 

 branous envelope.' This envelope 

 is at first very delicate, and is 

 filled with green and granular con- 

 tents ; by degrees the envelope ac- 

 quires increased thickness, and the 

 contents of the spore-cell become 

 brown or red. The surface of the sporangium, as this body is 

 now termed, is sometimes smooth, as in Closterium and its allies 

 (Fig. 76), and in the Desmidiecp proper (Fig. 77) ; but in the 

 Cosmariece, it acquires a granular, tuberculated, or even spinous 

 surface (Fig. 75), the spines being sometimes simple and some- 

 times forked at their extremities.^ 



170. The mode in which conjugation takes place in the fila- 

 mentous 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 con- 



' In Closterium linealvm, as in many of the Diatomacece (§ 178), the act of conjugation 

 gives origin to two sporangial cells. 



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

 kind, are often fonnd fossilized in flint, and have been described by Ehrenberg under 

 the name ot XuiUhidia. 



Cosmarium botryiis: — a, mature frond; 

 H, empty frond ; c, transverse view j jj, 

 sporangium with empty fronds. 



