ZYGOSPORES. 



'^Sl 



3. Ulothrichace^. Differing in many respects from the Pandorineae and Hydrodic-' 

 tyeae is the genus Ulothrix, an Alga consisting of segmented filaments composed of cells 

 which are all alike. It is mentioned in this connection only because it is characterised 

 by the conjugation of equivalent zoogonidia. Cramer states^ that the contents of 

 some of the individual cells of a filament break up into two, four, or eight zoogonidia 

 which immediately germinate and produce new filaments. Other of the cells, on the 

 other hand, give birth to 16 or 32 microzoogonidia which, after escaping, conjugate 

 exactly like those of Pandorina. Nothing is known of the development of the zygo- 

 spores. It is questionable whether Ulothrix is not more nearly related to Sphceroplea 

 among the Oosporeae, and whether the conjugation of the zoogonidia is not to be 

 regarded as a simpler case of the formation of oospores which occurs in the latter 2. 



B. Conjugation takes place betiveen stationary cells. 



(a) The CoNjUGATiE ^ consist of cells with a limited power of growth, which 

 lultiply to an unlimited extent by bipartition ; the cells thus produced either live 

 mtirely independently, or remain united in filaments. The chlorophyll-bodies form 

 jither parietal bands, axile plates, or radiate bodies arranged in pairs. Conjugation 

 takes place between ordinary vegetative cells, the contents coalescing in a variety of 

 'ways, the resulting zygospore becoming invested with a cell-wall, germinating after 

 a period of repose, and presenting essential differences in form from that of the 

 vegetative cells. There are no distinct gonidia, the ordinary vegetative cells per- 

 forming the functions of reproductive organs. De Bary divides the Conjugatae into 

 three families : — 



(i) The MesocarpeeB consist of cylindrical segmented filaments with an axile plate 

 of chlorophyll ; filaments which lie parallel to one another put out conjugative processes, 

 or two cells of contiguous filaments come into contact by knee-like projections ; the parts 

 of the walls which are in contact become absorbed and a broad canal is formed in 

 which the protoplasm of the two conjugating cells collects; the canal then becomes 

 shut off by two or four transverse septa and constitutes the zygospore. This mode of 

 production of zygospores is clearly analogous to similar processes in the Zygomycetes. 

 On germination the zygospore produces at once a segmented filament, the end that 

 remains in the spore forming its base and the exposed end its apex. This contrast is 

 not, however, permanent ; all the cells subsequently multiply by transverse division. To 

 this family belong the genera Mesocarpus, Craterospermum, and Staurospermum. 



(2) The ZygneraeaB consist also of cylindrical segmented filaments with the 

 chlorophyll arranged in straight or spiral parietal bands or in stars placed in pairs. 

 Conjugation takes place between two parallel filaments; the individual cells put 

 out opposite conjugative protuberances (see Figs. 5 and 6, p. 10) which eventually 

 touch one another, when the absorption of the cell-walls at the point of contact 

 forms a narrow canal. Since a number of cells of two filaments usually conjugate 

 at the same time, the whole forms a ladder-like structure in which the rungs are 

 represented by the canals. After the formation of the conjugating canal the proto- 



^ Cramer, Naturfor, Gesellsch. in Zurich, March 21st, 1870. 



^ [A. Dodel (Jahrb. fiir wiss. Bot. 1876, vol, X. Heft 4) has described in detail the germination 

 of the zygospores of Ulothrix. The whole course of development presents a striking analogy to 

 that of Hydrodictyon. Dodel has, like Areschoug, observed occasional conjugation between the 

 microzoogonidia ; if these do not conjugate, they then propagate themselves non-sexually like the 

 macrozoogonidia.] 



^ De Bary, Untersuchungen iiber die Familie der Conjugaten, 1858. [See also Hassall, Hist. 

 Brit. Freshwater Algoe, 1845; Wittrock on Mesocarpese ; Quart. Journ. Micr. Sc. 1873, p. 123. 

 Besides the modes of conjugation described in the text, contiguous cells of the &ame filament conju- 

 gate by lateral processes both in MesocarpecB and Zygnemece.] 



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