50 



FIRST GROUP. — THALLOPHVTES. 



at the meeting-point form a somewhat narrow canal. Since several cells of a filament 

 usually conjugate simultaneously, they form altogether a ladder-like structure, in which 

 the conjugation-canals represent the rungs. When the canals are formed, the proto- 

 plasmic bodies of the two cells contract, and one of them glides over to the other 

 through the canal and unites with it to form a rounded zygospore, which invested with 

 a thick many- shelled wall remains lying within the much broader mother-cell-wall and 

 does not germinate till it has passed through a long period of rest (Fig. 27) : here too 

 there is at first a distinction of base and apex in the young plant, but this distinction 

 disappears at a later period, as in the Mesocarpeae, since all cells are alike in form and 

 behaviour during the time of vegetation. The genera Zygnema, Spirogyra, Moiigeolia, 

 Sirogoniian, Zygogomu//i belong to this family. 



3. The Desmidieae^ consist of cells that live isolated, or less frequently in rows 

 which readily break up into separate cells and are embedded in mucilage. The cells 

 are cylindric or fusiform, and sometimes have horn-like processes ; or their general out- 

 line is circular or elliptic, divided by a deep constriction into symmetrical halves. 

 Where there is no constriction, the chlorophyll-body in the interior of the cell is sym- 

 metrically halved, or else the symmetry is indicated by the so-called amylum-bodies 

 and the distribution of the starch-grains. It is in accordance with this symmetrical 

 formation that in the vegetative multiplication of the cells (individuals) a dividing wall 

 appears in the plane of symmetry (or within the constriction when there is one) ; this 



KiG. 27. Germination of SpirOi;yra /u^'aiis. I resting zygospore. // commencement of germination of the same. /// older 

 germ-plant from a zygospore which was inclosed in the cell c of a filament, and in which the conjugation-tube is still to be seen ; 

 e outer membrane of the spore, y yellowish-brown layer, !,' third and innermost layer of the cell-wall of the spore forming the germ- 

 tube; 7inv' the first transverse walls of the germ-tube, the hinder extremity of which d grows out into a narrow process. After 

 Pringsheim in Flora, 1852 No. 30. 



wall parts into two lamellae, and thus the halves are separated ; a new half grows out 

 at the place of separation and a new individual is thus formed complete and symme- 

 trical like the original one. The mode of formation of zygospores is the same as in 

 the Zygnemeae ; but in the simplest cases, as in Cylindrocystis and Mesotaenhon and 

 others, where the conjugating individuals are of very simple form, the conjugation ap- 

 pears to be nothing more than a coalescence comparable with the pairing of the gametes 

 of Pandorina, &c. The zygospore either germinates directly or its contents produce 

 two or more daughter-cells, each of which exhibits the vegetative multiplication de- 

 scribed above. 



These processes may be illustrated by the case of Cosmarium Botrytis, as portrayed 

 by De Bary in Fig. 28. The cells live isolated, and are divided by a deep constriction 



^ There is really no difference but that of habit between the Desmideae and Zygnemeae, and not 

 that in all species ; the two families therefore cannot properly be separated from one another. 

 [Fischer, A., Ueber d. Zelltheilung d. Closterien (Bot. Ztg. 1883, p. 225).] 



