258 



THALLOPHYTES. 



plasmic bodies of the two cells contract, one of them passes through the canal into the 

 other, and the two coalesce into a round zygospore invested with a thick cell-wall 

 consisting of several layers which lies within the much larger mother-cell, and which 

 germinates only after a period of rest (Fig. 170). In the Zygnemeae there is also at first 

 a contrast in the young plant between base and apex, which afterwards disappears, all 

 the cells behaving exactly alike during growth. Among the genera are Zygnema, 

 Spirogyray Mougeotia, Sirogonium, and Zygogonium. 



(3) The DesmidiesB ^ consist of cells which live isolated or less often in rows which 

 easily break up and are imbedded in mucilage. The cells are either cylindrical or 

 fusiform, sometimes furnished with horn-like appendages {Scenedesmus) ; in other cases 

 they have a circular or elliptical outline, and are divided by a deep constriction into two 

 symmetrical halves. Even where there is no such constriction, the chlorophyll-body is 

 divided symmetrically in the interior of the cell, or the symmetry is indicated by the 

 so-called chlorophyll-vesicles [amylum-bodies] ^ and the distribution of the starch-grains. 

 In accordance with this symmetrical structure the vegetative propagation of the cells 

 (individuals) is effected by the formation of a septum in the plane of symmetry or 

 within the constriction which splits into two lamellae and divides the cell into two halves. 

 Growth at the point of separation then produces a segment which restores the symmetry. 

 The mode of formation of the zygospores is similar to that in the Zygnemeae ; but in the 



Fig. 170. — Germination of Spiroffyra jtigalis (after Pringsheini, Flora, 1852, no. 30 ; / a resting zygospore ; // commence- 

 ment of its germination ; /// the young plant further developed from a zygospore, which had been enclosed in the cell C, the 

 conjugating canal c being still visible; e outer cell-wall of the spore; y yellowish brown layer of the cell-wall ; g the third and 

 innermost layer of the cell-wall of the spore, which forms the germinating filament ; w 71/ the first septa of the germinating 

 filament, the posterior end d growing into a narrow appendage. 



simplest forms, as Cylindrocystis and Mesotceniurn, where the conjugating individuals are 

 of very simple form, conjugation appears to be nothing but a coalescence, altogether 

 similar to that of the zoogonidia of Pandorina. The zygospore either germinates 

 directly or divides into two or four daughter-cells, each of which repeats the vegetative 

 mode of reproduction already described. In the last-named case we may consider, as 

 in Pandorina, the zygospore to be a fructification which produces several spores in the 

 sense in which the term is used in the Muscineae; and we may here again have an 

 indication of an alternation of generations. 



These processes may now be explained more in detail in the case of Cosmarium 

 Botrytis (Fig. 171), as described by De Bary [I.e.). The cells live isolated, and are 

 symmetrically bisected by a deep constriction (Fig. 171, X), and are also compressed 



* [See also Ralfs, British Desmidieoe, 1848. — Archer in Pritchard's Infusoria.] 

 2 [Strasburger has shown (Zelltheilung iind Zellbildung, 2*® Aufl. p. 83) that the structure 

 existing in the chlorophyll-bodies of Zygnema and Spirogyra are true chlorophyll-granules in which 

 numerous starch-grains are placed concentrically so as to form a layer at the surface. These struc- 

 tures are identical with those mentioned above as chlorophyll-vesicles and amylum-bodies.] 



