176 



CRYPTOGAMS 



the protrusion of one or two tubular filaments, which 

 grow directly to new plants. 



425. Zoospores are apt to be formed when the plant is 

 growing in a submerged situation. In places where it is 

 exposed to the air and moistened only occasionally, as by 



the dew, a second method of repro- 

 duction prevails. Swellings arise 

 on the thallus, which develop into 

 short, thick branches of peculiar 

 form. When cut off by septa 

 below they become the oogonia 

 (Fig. 290, og'). The contents of 

 the oogonium contracts somewhat 

 to form the egg cell^ and an open- 

 ing makes its appearance in the 

 oogonium wall. Near by, short, 

 slender, often coiled branches grow 

 up. Their extremities are cut off 

 to form the antheridia (Fig. 290, a), 

 from which antherozoids, bodies 

 resembling small zoospores, are 

 finally liberated. The latter make their way through 

 water to the opening of the oogonium, and one, enter- 

 ing, fuses with the egg cell. The resulting body, or 

 oospore, now surrounds itself with a cell wall and enters 

 a resting state. It is ultimately set free by the rupture 

 of the oogonium wall, and germinates. 



290. Vaucheria: A, the un- 

 opened antheridium 

 (a) and oogonium 

 (og) ; B, the same 

 after fertilization 

 and formation of 

 the oospore (os) . — 

 Pringsheim. 



426. In Vaucheria we have essentially the same reproductive pro- 

 cesses as in Ulothrix, but now appearing in a much modified form. 

 The single large zoospore of Vaucheria, with its many cilia, performs 

 the same office as the numerous small zoospores of Ulothrix. The pro- 

 duction of the oospore in Vaucheria may be likened to the union of 

 reproductive cells in Ulothrix, with the important difference that now 

 the fusing- cells differ greatly in size, and only one of them is motile. 



427. Cells designed for reproductive union are called gametes. 

 When they are of unequal size, the larger is termed egg cell or simply 

 egg : the smaller, if motile, is an antherozoid. The e.gg is said to be 

 fertilized by the antherozoid. The body directly resulting from the 

 union of unequal gametes is an oospore. 



