156 STRUCTURAL BOTANY 



after the protoplasm of the spermatozoid has united 

 with that of the female cell, the nucleus of the former 

 can still be distinguished. It is much smaller than that 

 of the ovum. The male nucleus passes through the 

 protoplasm until it reaches the female nucleus, and then 

 the two unite to form a single nucleus. Thus even in so 

 simple an Alga as this, we see that fertilisation is in 

 essentials precisely the same process as in the highest 

 Phanerogams, such as the Lily. 



The fertilised ovum (which is now called the oospore) 

 contracts further, surrounds itself with a cell-wall, and 

 gradually passes into a resting state. The contents 

 undergo great changes ; the chlorophyll disappears and 

 is replaced by a brown or red colouring matter, while 

 large quantities of oil appear in the protoplasm, and at 

 the same time the cell-wall becomes much thickened. 

 The resting stage may only last for a few weeks. It 

 appears that in some at least of the species germination 

 takes place before winter comes on. In the mean time 

 the oospore has remained enclosed within the oogonial 

 wall. 



When germination begins, the inner layers of the cell- 

 wall of the oospore swell, and burst the hard outer 

 coat ; the entire contents surrounded only by a delicate 

 membrane now become free, leaving behind both the 

 outer oospore-wall and that of the oogonium. In the 

 normal course of development the oospore does not 

 immediately give rise to a new plant; its contents 

 divide into four cells, each of which rounds itself off and 

 becomes a ciliated zoospore, exactly resembling the 

 zoospores formed in the vegetative cells, except that the 

 contents are wholly or partly of a red colour (see Fig. 

 69, C). These zoospores free themselves from the 



