EEPRODUCTION 



425 



The gametangia of the plants above the Thallophytes 

 are known as antheridia and archegonia respectively. An 

 archegonium is a more complex structure than an oogonium, 

 being composed of many cells and showing differentiation 

 into a venter and a neck (fig. 173). It contains only a 

 single oosphere. 



The sexual cell differ from the great majority of 

 asexual ones in never possessing cell -walls. The only 

 cases in which they are clothed with them are those of the 

 Ehodophycece and the Ascomycetes already alluded to. In 

 both these groups the male gametes are the only ones that 



FIG. 173. DEVELOPMENT OF THE ARCHEGONIUM OF THE FERN. 



have them ; the females, as we have seen, not being 

 differentiated. 



The fusion of the gametes is known as conjugation 

 when they are alike, and as fertilisation when they are 

 distinctly male and female. The resulting body is termed 

 a zygote ; it is a zygospore when it is produced by conjuga- 

 tion, and an oospore when it is the result of fertilisation. 



In the more lowly organised forms it generally happens 

 that both sexual and asexual reproductive cells may be 

 produced upon the same individual. An exception is 

 found in the Fucacece, the members of which do not 

 develop any asexual cells. While it is possible, however, 

 for many plants to produce both gonidia and gametes, it is 



