270 



THALLOPHYTES. 



the branching dichotomoiis; though commencing monopodially, the lateral branches 

 often develope sympodially. At the commencement of the period of growth in the 

 spring the plants arise from the oospores which have remained dormant through the 

 winter, and are at first propagated through several generations in a purely non-sexual 

 manner. This is effected either simply by the abstriction of the ends of branches, or by 

 very large zoogonidia which escape from the interior of a cell, and are covered by very 

 short cilia. Between these two forms the various species of Vaucheria show intermediate 

 steps. In V. tuberosa, for example, branches swell up to a considerable size, become 

 detached at the base, and put out at once one or more germinating tubes. In 

 V. geminata the end of a branch swells up to an oval shape, its contents become 

 separated by a septum, contract, and form a new cell-wall, and the gonidium thus 

 formed either becomes free by the decomposition of the mother-cell-wall, or falls off 

 along with it, germinating after some days. The gonidia of F. hamata are formed in 

 the same way, but are thrown out with a jerk, remain at rest, and germinate 

 during the next night. In other species, as V. sessilis, sericea, and piloboloides, the 



Fig. 176. — Vaucheria sessilis (X about 30). 



contents of a branch become separated and contract, and force themselves out as 

 a zoogonidium through a crevice at the extremity. The motion of the zoogonidia lasts, 

 in V. sericea^ only for ^ to i^ minutes, but in other cases for hours. The rotation 

 begins, in V. sessilis, as I have distinctly seen, during their escape ; and if the opening 

 of the mother-cell is too small, the zoogonidium splits into two pieces ; each becomes 

 rounded off and again constitutes an entire zoogonidium; the outer one swims away, 

 while the inner one remains rotating within the mother-cell. The formation of the 

 zoogonidia begins in the night, as is the case with most Algae and Fungi; they escape in 

 the morning, and their germination commences during the day or the next night. 

 They put out on germination either only one or two tubes (Fig. 176, C, Z)), or form at 

 the same time a root-like organ of attachment (-B, F, wS). The oogonia and antheridia 

 originate as lateral protuberances from a filament which contains chlorophyll (Fig. 177,^, 

 5), sometimes even on the germinating tube of a zoogonidium. All the species of Vaucheria 

 are monoecious, and the two kinds of sexual organs are mostly found very near together. 

 The antheridia (Fig. 177, ha) are the terminal cells of slenderer branches, the contents 



