CRYPTOGAMS 



317 



function of the zygospore to act as a resting-spore, to tide over the 

 winter or a period of drought, and eventually, on germination, to give 

 rise to a new filament of Spirogyra. This form of conjugation, which 

 is the one peculiar to most species, is described as scalariform (Fig. 

 236, A), as distinct from the lateral conjugation of some species, in 

 which two adjacent cells of the same filament conjugate by the 

 development of coalescing processes, which are formed near their trans- 

 verse wall (Fig. 236, B). 



2. Mesocarfaceae. — The representatives of this family are also composed of 

 filamentous rows of cells, but exhibit a difference in their mode of conjugation. 

 In this case, in the process of conjugation, which is either scalariform or lateral, 

 only a portion of the protoplasm of both conjugating protoplasts, together with 

 their nuclei and a greater part of their chromatophores, passes into the connecting 

 canal, and there, fusing into a zygospore, becomes separated from the parent cells 

 by transverse walls. 



3. In the Desmidiaceae, the third family of the Conjugatae, are comprised the 



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ki^iii 



;- . ' 



~'rC>! : 



^i§? 



s 



Fir.. 237 



Cosmarium coelaium in process of 

 division ; B, Cosmarium Botrytls ; C, the same 

 with fully-developed zygospore ; D, Mierasicrias 

 CruxmalitenHs. (After Ralfs.) 



Fig. 238. — Closterium moniliferum ; p, 

 pyrenoid ; K, vesicle with crystals. 

 (X 240.) 



unicellular forms. They are ornamented with delicate markings, and, like the 

 Diatoms, exhibit a great variety of form (Figs. 237, 238). Their cells are composed 

 of two symmetrical halves, separated, as a rule, from each other by a deep 

 constriction, the isthmus. Each half contains a large, radiate, irregularly 

 defined chromatophore, or a number of plate-like chromatophores united into 

 one. Within the chromatophores are disposed several pyrenoids, while the 

 nucleus lies in the centre of the cell in the constriction. The cells themselves 

 display a great diversity of form and external configuration (Figs. 237, 238). The 

 cell walls are frequently beset with wart- or horn-like protuberances. In some 

 cenera there is no constriction between the two halves of the cell. This is the 

 case, for instance, in the crescent-shaped Closterium moniliferum (Fig. 238), whose 

 two chromatophores consist of six elongated plates, united in the long axis of the 



