178 STRUCTURAL BOTANY 



the soil by a colourless rhizoid, and soon begins to 

 branch (see Fig. 75, B). 



This mode of reproduction may go on indefinitely, 

 from generation to generation, as long as weather and 

 water remain favourable. But under other circum- 

 stances, and especially if the level of the water is 

 sinking, and a danger of drought threatens, Vaucheria, 

 proceeds to form reproductive bodies of another kind. 

 This plant, so simple in its vegetative structure, 

 possesses more sharply differentiated antheridia and 

 oogonia than any of the Algse already described. Both 

 organs arise as lateral outgrowths. In some species 

 they are seated directly on the main filament ; in others 

 a short special branch forms a pedicel on which the 

 sexual organs are borne, as shown in Fig. 76, where the 

 oogonium is terminal on the pedicel, while the anther- 

 idium is seated laterally below it. 



The antheridium at its first origin resembles a 

 young vegetative branch. It contains abundant 

 protoplasm with chloroplasts and very numerous 

 nuclei. As it approaches maturity, it usually becomes 

 curved like a horn (Fig. 76). The terminal part is cut 

 off from the rest by a transverse wall, and forms the 

 true antheridium. The nuclei assemble in the interior 

 of the cell, leaving the chloroplasts and part of the 

 protoplasm in contact with the wall. The more 

 internal portion of the contents now breaks up into 

 the excessively minute spermatozoids, each of which 

 consists of one of the nuclei, together with an ex- 

 tremely small quantity of protoplasm, and is provided 

 with two cilia. They begin to swarm within the 

 antheridium, which now opens at the apex, and expels 

 the spermatozoids, together with a portion of the 



