218 



ZOOSPORES 



cut off by separating walls to form so-called zoosporangia 

 (Fig. 118, B). Within each the contents round off and the 

 numerous nuclei take up a position outside the chloroplasts. 

 A pair of cilia arise opposite each nucleus, so that a large multi- 



ciliate oval zoo- 

 spore (Fig. 118, 

 C) is formed, 

 which escapes 

 into the water 

 by the breaking 

 down of the tip 

 of the sporan- 

 gium. Its move- 

 ments are slow 

 and usually cease 

 after a short 

 time with the 

 withdrawal of 

 the cilia ; a thin 

 wall is formed 

 and the two ends 

 lengthen into 

 tubes, of which 

 one frequently 

 penetrates the 

 soil or mud and 

 becomes a colour- 

 less attaching 



FIG. 118. Vaucheria. A, Portion of a plant show- organ (Fig. Il8, 

 ing the branched non-septate thallus and the 

 colourless rhizoid-like attaching organ (r.). 

 B, Zoosporangium. C, Zoospore. D, Small 

 part of thallus, showing the numerous chloro- 

 plasts (c.), and nuclei (n.). E, Germinated 

 zoospore. (C after Oltmanns ; rest original.) 



E, r). 



Zoospores are 

 also encountered 

 in some of the 

 Brown Algae, e.g. 



in Ectocarpus 



and Laminaria. In the former they develop in considerable 

 numbers in oval sporangia, each borne laterally on a branch 

 of the filament (Fig. 119, c), and liberating its contents by 

 rupture of the apex. The pear-shaped zoospores have two 



