154 



BOTANY 



Vaucheria. The zoospores are produced in great numbers, by the division of the 

 protoplasm of the sporangium into as many parts as there are nuclei present. 



In Saprolegnia they es- 

 ^\ C D cape from a terminal pore, 



^ and are pear-shaped with 

 two terminal cilia. They 

 usually become encysted, 

 and escape from the cysts 

 as bean-shaped zoospores 

 with two lateral cilia. 

 These, on coming to rest, 

 send out a germ-tube and 

 grow into a new plant. 

 In other genera (e.gr., 

 Achlya, Aphanomyces), 

 the zoospores become en- 

 cysted immediately upon 

 escaping from the spo- 

 rangium (E). In Dicty- 

 uchus the sporangium is 

 divided into chambers in 

 each of which a zoospore 

 is produced. 



After the sporangium 

 is empty, a new one is 

 formed, either by the end 

 of the filament growing 

 through the empty spo- 

 rangium (Fig. 118, D), or 

 by pushing out below its 

 base and forming a new 

 sporangium by the side 

 of the old one. This 

 process may be repeated 

 several times. 



sp 



FIG. 118. A-D, Saprolegnia ferax. A, dead fly 

 covered with Saprolegnia. B-D, development of 

 zoosporangium (x 225) ; in D, a new sporangium has 

 formed within an empty one. E, Achlya sp., sporan- 

 gium discharging the encysted zoospores; sp, young 

 sporangium developing below the empty one (X 175). 

 F, Aphanomyces sp., free zoospore. G, Saprolegnia 

 sp., zoospore (X 475). (F, G, after HUMPHREY.) 



Sexual Reproduction. Ob'gonia, globular in form, are found in most species. 

 Within the oogonium one or several egg-cells are formed (Fig. 119). The anther- 

 idium is smaller than the oogonium and formed usually from another branch. In 

 S. hypogyna, the antheridium develops from the same branch as the oogonium. 

 The antheridia are irregular in form, and there may be a single one, or several, 

 formed in connection with each oogonium. The antheridium is closely applied 

 to the wall of the oogonium, into which it sends a tube, which comes into con- 

 tact with the egg-cell, into which, in some cases, e.g. S. dioica, a nucleus passes 

 from the fertilizing tube, and fuses with the single nucleus in the egg-cell. In 

 most species examined, however, it has been found that although the fertiliz- 

 ing tubes are present, they are not functional, and the oospores are formed with- 

 out fertilization. Still other forms, e.g. S. ferax, seldom or never produce 

 antheridia. 



The ripe spores, after a period of rest, germinate by sending out a 

 germ-tube which penetrates the nutritive body. 



Kelated to the Water-moulds is the genus Pythium, some species 

 of which are very destructive parasites. One of these, P. De Barya- 



