THE ALG^E 181 



which it unites. It appears then that in Vaucheria, 

 in spite of the remarkable peculiarities of its organisa- 

 tion, fertilisation is precisely the same process as in the 

 higher plants. 



The fertilised ovum surrounds itself with a cell-wall 

 of some thickness, and passes into a resting-stage 

 (see Fig. 76, D) during which a period of drought 

 can be endured with safety. Then germination 

 takes place ; the oospore gives rise directly to a new 

 Vaucheria plant, which soon begins forming the asexual 

 zoospores. 



We see that there is in this plant no indication of 

 a regular alternation of generations. Sexual and 

 asexual reproduction may both occur in the same 

 individual, and it depends on the external conditions 

 whether the one or the other takes place. 



The sexually produced resting-spore is itself a pro- 

 vision against drought, but the plant can also protect 

 itself against this danger in a more rough and ready 

 fashion. If the water sinks and the filaments are left 

 stranded on the mud, it sometimes happens that they 

 divide up into a number of cells, each of which surrounds 

 itself with a thick wall. This is called the Gongrosira 

 state, because specimens of Vaucheria in this condition 

 used to be placed in a different genus under that name. 

 We see then that Siphonaceous filaments can separate 

 into distinct cells when necessary, though this does 

 not happen in the normal course of their vegetative 

 life. 



Vaucheria stands quite alone among its relatives 

 in the perfection of its reproductive arrangements. In 

 other Siphonese, in which any sexual process has been 

 observed, it takes the form of the conjugation of motile 



