268 THE ALGAE 



is motile for about fifteen minutes, after which it comes to rest and 

 germinates, the first thread often being more or less colourless. 

 ^Zoospore' formation can often be induced by transferring the 

 plants from Hght to darkness, or from a nutrient solution to distilled 

 water. 



Under dry conditions aplanospores may be formed at the ends of 

 short laterals or terminal branches, whilst if exposed to greater 

 desiccation the threads of the terrestrial forms become septate and 

 rows of cysts are formed, thus giving the *Gongrosira' stage. When 

 conditions become more favourable these cysts germinate either 

 into new filaments or else into small amoeboid masses which grow 

 into new filaments. Sexual reproduction is distinctly oogamous, 

 the different species being either monoecious or dioecious. The 

 oogonia, which are sessile or stalked, are cut oif by a septum at a 

 stage when there is only one nucleus left in the oogonium. Some 

 authors maintain that the extra nuclei, which are potential gametes, 

 degenerate, whilst others consider that the surplus nuclei, enclosed 

 in a mass of cytoplasm or Vanderplasm', travel back into the main 

 thread before the septum is laid down. It is probable that in some 

 species all the surplus nuclei pass out with the Vanderplasm', 

 whilst in other species some nuclei may be left behind and degene- 

 rate later after the septum has been laid down. The factors that 

 determine the selection of the functional nucleus from among the 

 number available offer a problem for future research. It would ap- 

 pear that in some cases at least, the loss of the nuclei may be as- 

 cribed to an unfavourable environment. In the mature oogonitun 

 there is either a beak, the apex of which gelatinizes, or else several 

 pores through which the antherozoids can enter the oogonium, 

 fertiUzation taking place in situ. 



The antheridia, which are usually stalked, commonly arise close 

 to the oogonia, though in V. sessilis they develop just prior to 

 oogonial formation. When the septum cutting off the antheriditmi 

 is laid down the nuclei divide, and cytoplasm gathers around each 

 daughter nucleus. The mature antheridium may be colourless 

 or green, and it opens by one or more apertures near the apex, thus 

 providing a means of escape for the pear-shaped antherozoids 

 which bear two flagella pointing in opposite directions. After 

 fertilization the zygote develops a thick wall and remains dormant 

 for some time before it germinates to give rise to a new filament. 

 The latest evidence shows that reduction of the chromosome 



