234 



THALLOPHYTES. 



1. Conjugation and Production of Zygospores. Two cells of similar if not 

 always of precisely the same nature coalesce, and produce a reproductive cell 

 termed a Zygospore^ which germinates after a shorter or longer period of rest, and 

 then gives rise either to spores or at once to a plant of the same kind as that in 

 which the conjugation took place. An alternation of generations is exhibited only 

 in so far as the zygospore constitutes the entire second (non-sexual) generation. 



The process of the formation of zygospores has a very different appearance 

 according to the nature of the conjugating cells. The simplest case is presented 



in the conjugation of zoogonidia 

 A r, discovered by Pringsheim (Fig. 



162, A). These bodies during 

 the process of swarming come 

 into contact in pairs by their 

 hyaline anterior ends, and then 

 gradually coalesce into a primor- 

 dial cell, which subsequently be- 

 comes invested with a cell-wall, 

 and then grows, producing again 

 motile cells, and each of these 

 gives rise to a plant of the ori- 

 ginal kind. These Zoospores which 



Fig. 162.— Various forms of conjugation and the production of zygospores; rCSUlt frOm thc ZVG'OSDOrC maV 

 A conjugation of the zoogonidia of Panciorina ; H formation of zygospores / o x^ / 



in Piptocephalis (after Pringsheim and Brefeld). The numbers indicate in '^g COnsldcrcd aS trUC SDOrCS in 

 each case the successive stages of development. ' ^ 



the same sense as those of the 

 Muscineae; for the zygospore is homologous with the spore-capsule of Mosses, 

 and represents a rudimentary alternate generation. The conjugation of Spirogyra, 

 as illustrated in Fig. 6, p. 10, is somewhat more complicated. The conjugating 

 cells are here surrounded by a firm cell-wall ; they put out protuberances opposite 

 to one another, which unite to form a canal, through which the contents of 

 one cell pass over into the other, and coalesce with its contents; the resulting 

 protoplasmic body invests itself with a cell-wall, and becomes a zygospore, which 

 again produces a Spirogyra filament by direct germination. The formation of 

 zygospores in the Zygomycetes is represented in Fig. 162, B. Here the two cells 

 which coalesce after having grown towards each other are perfectly alike and immotile ; 

 and it is only a portion of the coalesced contents which becomes separated by a 

 partition-wall, and produces the thick-walled zygospore which germinates after a 

 periodjof rest. 



2. The Formation of Oospores in Oogonia. The two reproductive cells are 

 here essentially different ; the female cell ox Oosphere is always a naked immotile 

 primordial cell developed within an older cell which is termed the Oogonium. 

 The male cells, the Antherozoids, the mother-cells of which are called Antheridia^ 

 are very small, and are endowed with motion by means of vibratile cilia; they 

 swarm round the oosphere, and cause its impregnation by the coalescence of their 



De Bary, Thuret, Nageli, Janczewski, Brefeld, and others ; though a different signification to 

 ^that of the authors is sometimes applied to them. 



