408 



BOTANY 



PART II 



of the sporangium then ruptures at the apex, and the swarm-spore, rotating 

 on its longitudinal axis, forces its way through the opening. The swarm -spore 

 is so large as to be visible to the naked eye, and contains numerous nuclei 

 embedded in a peripheral layer of colourless protoplasm. It is entirely surrounded 

 with a fringe of cilia, which protrude in pairs, one pair opposite each nucleus. 

 Morphologically the swarm -spores of Vaucheria correspond to the total mass of 

 individual zoospores of an ordinary sporangium. 



The sexual reproduction of Vaucheria is not effected, like that of the other 

 Siphoneae, -by the conjugation of motile gametes, from which, however, as the 

 earlier form of reproduction, it may be considered to have been derived. The 

 oogonia and antheridia first appear as small 

 protuberances, which grow out into short lateral 

 branches, and become separated by means of 

 septa from the rest of the thallus (Fig. 348 o, a). 

 At first, according to OLTMANNS and HEIDINGEK, 

 the rudiment of an oogonium contains numerous 

 nuclei, of which all but one, the nucleus of the 

 future egg-cell, retreat again into the main 

 filament before the formation of the septum. 

 In its mature condition the oogonium has on 

 one side a beak-like projection containing only 



FIG. 348. Vaucheria sessilis. Portion of a filament 

 with an oogonium, o ; antheridium, a ; ch, chro- 

 matophores ; n, cell nuclei ; ol, oil globules, 

 (x 240. After STRASBURGER.) 



Fio. 349. Botrydinm granulatum. 

 A , The whole plant. B, Swarm- 

 spore. (A x 28 ; B x 540. After 

 STRASBURGER.) 



colourless protoplasm. The oogonium opens at this place, the oosphere rounding 

 itself off. The antheridia, which are also multinucleate, are more or less coiled (a], 

 and open at the tip to set free their mucilaginous contents, which break up into a 

 number of swarming spermatozoids. The spermatozoids are very small, and have 

 a single nucleus and two cilia inserted on one side. They collect around the 

 receptive spot of the egg-cell, into which one spermatozoid finally penetrates. 

 After the egg-cell has been fertilised by the fusion of its nucleus with that of 

 the spermatozoid, it becomes invested with a wall and converted into a resting 

 oospore. On germination the oospore grows into a filamentous thallus. 



Botrydium granulatum (Fig. 349), which was formerly included in the Hetero- 

 contae, may be placed in the Siphoneae. This Alga is cosmopolitan and grows 

 on damp clayey soil, where it forms groups of green balloon-shaped vesicles about 

 2 mm. in size. These are attached to the soil by branched colourless rhizoids. The 

 whole plant corresponds to a single multinucleate cell ; its protoplasm contains 

 numerous green chloroplasts. The zoospores, produced in large numbers by the 



