DIV. I 



THALLOPHYTA 



431 



the mycelium (Figs. 372, 373). The oogouia give rise to a larger (as many as 50) 

 or smaller number of egg-cells, rarely only to a single one. At first the oogonium 

 contains numerous nuclei, most of which, however, degenerate ; the remaining 

 nuclei divide once mitotically into daughter nuclei, of which some again degenerate, 

 while the oospheres become delimited around the remaining nuclei. The egg-cells 

 are always uninucleate. The tubular antheridia, with a number of nuclei that undergo 

 one mitotic division, apply them- 

 selves to the oogonia and send 

 fertilising tubes to the egg-cells. 

 One male nucleus enters the egg- 

 cell and fuses with its nucleus. 

 The oospore after fertilisation 

 acquires a thick wall. The re- 

 duction division takes place on 

 the germi nation *of the oospore. 

 In some forms belonging to this 



The 



FIG. 371. Xcii'i-oh'-jiiM m 



biciliate zoospores, s-, are escaping 

 from the sporangium. (After G. 

 KLEBS.) 



FK;. 372. Saprolegnia mixta. Hyphae bearing the 

 sexual organs : a, antheridium which has sent 

 a fertilisation tube into the oogonium ; oi, 

 egg-cell ; o 2 , oospore enclosed in a cell wall ; op, 

 parthogenetic oospores ; g, young oogonium. 

 (After G. KLEBS.) 



family, and also in some Peronosporeae, the formation of antheridia is occasionally 

 or constantly suppressed ; the oospores develop parthenogenetically without being 

 fertilised (Fig. 372 op). 



3. The Peronosporeae ( 56 ) are parasitic fungi whose mycelium penetrates the 

 tissues of the higher plants. In damp climates certain species occasion epidemic 

 diseases in cultivated plants. Thus, the mycelium of Phytoplitliora infestans, the 

 fungus which causes the Potato disease, lives in the intercellular spaces of the 

 leaves and tubers of the Potato plant, and by penetrating the cells with its short 



