ZYGOSPORES. 
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 Vlothrix 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^. 
B. Conjugation takes place betaueen stationary cells. 
(a) The Conjugate ^ consist of cells with a limited power of growth, which 
multiply to an unlimited extent by bipartition ; the cells thus produced either live 
entirely independently, or remain united in filaments. The chlorophyll-bodies form 
either 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 Conjugatge into 
three families : — 
(1) The MesocarpesB 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. T his 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 Zygnemeae 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 Zürich. March 21st, 1870. 
^ [A. Dodel (Jahrb. für 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 über die Familie der Conjugaten, 1858. [See also Hassall, Hist. 
Brit. Freshwater Algse, 1845; Wittrock on Mesocarpeae ; Quart. Journ. Micr. Sc. 1873, p. 123. 
Besides the modes of conjugation described in the text, contiguous cells of the same filament conju- 
gate by lateral processes both in Mesocarpece and Zyg?iemecB.'\ 
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