434 



VEGETABLE PHYSIOLOGY 



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 cells 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 Rhodo- 

 phycece and the Ascomycetcs already alluded to. In both 

 these groups the male gametes are the only ones that 



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



1, 2, 4, 5, 7, 8, 9, Successive stages. 3, 6 transverse sections of the 

 neck region of 4 and 5. 



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 conjugation, 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 



