THE FUNGI 



225 



into the air. An oogonium arises as a spherical swelling 

 on a hypha, and may be either terminal, as shown in 

 our Fig. 95, or intercalary, i.e. produced at some in- 

 termediate point in the course of the filament. The 

 young oogonium is cut off' from the rest of the hypha by 

 a transverse cell-wall. 



Its protoplasm now separates into two parts, a 

 central granular portion which becomes the ovum, and a 

 peripheral layer, lining the cell-wall, called the periplasm. 

 The behaviour of the nuclei has not yet been made 

 quite clear in Pythium, but there 

 is no doubt that the oogonium 

 at first contains a large number 

 of nuclei; judging from the 

 analogy of allied genera, it is 

 probable that nearly all of them 

 pass into the periplasm, leaving 

 behind, in the central mass, a 

 single nucleus, which is alone 

 concerned in the act of fertilisa- 

 tion. 



In the mean time the anther- 

 idium is formed. It is usually a 

 lateral, club-shaped branch, arising 

 either from the same filament which bears the oogonium 

 (see Fig. 95) or from a different one, and separated from 

 the hypha on which it is borne by a transverse wall. 

 The antheridium directs its growth towards the neigh- 

 bouring oogonium, to which it closely applies itself. 



It may be mentioned here that the mycelium of 

 Pythium and its allies, which is non-cellular during its 

 vegetative growth, generally becomes irregularly parti- 

 tioned up, by a few scattered transverse walls, as the 



15 



FIG. 95. Fertilisation of 

 Pythium. A, early stage ; 

 oogonium (o) and anther- 

 idium (a) still immature. 

 B, moment of fertilisation. 

 The contents of the 

 anthevidium (a) are pass- 

 ing through the fertilising 

 tube, to unite with the 

 ovum (o). Magnified 800. 

 (After De Bary.) 



