OF THE DESMIDIEiE. 13 



increase much in number, and, together with the granular contents, become 

 aggregated about the conjugating part. When contact is complete, an absorp- 

 tion of the opposed walls of the two cells takes place, or the suture of each 

 opens, the endochrome from both is discharged and intermingled, and an 

 orbicular green granular mass, enveloped in a mucous sheath thrown out 

 around it by the conjugating cells, is produced. When the process has pro- 

 ceeded thus far, the original valves are more or less completely emptied of 

 theii' contents, lose their vitality, and are sooner or later detached, and float 

 away from the sporangium developed. 



The foi-mation of a sporangium by conjugation occupies no great time. 

 Indeed, in the case of Closterium Ehreyibergii, the Eev. W. Smith tells us 

 that " the discharge of the endochrome and the fomiation of the sporangia 

 are accomplished with much rapidity, and may often be seen taking place in 

 the field of the microscope ; the whole operation not occupying more than 

 a few minutes .... During the formation of the sporangia there appears to 

 be a second development of mucus in the form of rings around the reproduc- 

 tive bodies ; this is probably only the eftect of the pressure produced by the 

 growth of the sporangia on the mass of investing mucus." 



This act of conjugation admits of slight variations in character, determined 

 by the form of the conjugating cells, and by other circumstances pecuhar to 

 the tribe, family, or genus in which in it occurs. In the filamentous species 

 of Desmidieae, the joints, as before noted, usually become separated before 

 their conjugation ; and in most instances the old valves left empty after the 

 act of conjugation are almost immediately detached from the sporangium ; 

 but in a few species they persist some time afterwards, e. g. in several of 

 Penium. In Didymoprium the separated joints, on conjugating, unite by 

 means of a narrow process pushed out from each, and often of considerable 

 length ; through this the endochrome of one cell is transferred into the other, 

 and thus the spoi-angium is produced within one of the two cells, just as in 

 the Conjugatae. In Staurastrimi and Micrasterias the contents of both 

 fronds are discharged into a delicate intermediate sac or bag, which gradually 

 thickens, produces eminences, and at last forked spines (II. 25). Again, in 

 Tetmemorus, " the process of foiming the sporangium (says Mr. Ealfs) is 

 interesting, as it exhibits a striking similarity to the change during the 

 formation of similar bodies in Stcmrocarpus among the Conjugatae. In 

 Staurocarpus, after conjugation, a subquadrate ceU is formed, within which 

 the endochrome is collected. The latter is at fii'st of the same figure as the 

 ceU, but in at least one species is at length condensed into a compact 

 globular body, and in every species the cell with the contained sporangium 

 finally separates from the filaments with which it is connected. In this 

 separate state I can discover no character by which to distinguish the 

 sporangium of Tetmemorus from one belonging to a species of Staurocarpus.''^ 

 To quote the same authority, — ^^In Penium Jenneri the conjugating fronds do 

 not open and gape at the suture, as is usual in the Desmidieae, but couple by 



small and distinct cylindrical tubes like many of the Conjugatae In 



Closterium two fronds unite by means of projections arising at the junction of 

 the two segments, and then the newly-formed portion continues to enlarge 

 until the original segments are separated by a cell of an irregular figure (II. 

 5, 6). The contents of the fronds being collected in this cell become a dense 

 seed-like mass, which is sometimes globular, resembhng the sporangium of 

 Mougeotia, and sometimes square, like that of Staurospermum. The newly- 

 formed cell is thinner and generally paler than the segments of the fronds ; 

 in some species it looks like a prolongation of the segments, and in others 

 these are so loosely attached that their connexion is scarcely perceptible. 



