224 STRUCTURAL BOTANY 



growing on Scrophulariacese, the protoplasm of the 

 sporangium is expelled entire through the apical opening 

 without dividing into zoospores ; it surrounds itself with 

 a new cell-wall, and germinates directly, to form the 

 mycelium of the next generation. In P. Laetucce (Fig. 

 94, C) (which infests Lettuces and their allies) a further 

 step is taken ; the contents do not escape at all, but the 

 sporangium simply puts out a hypha which arises at the 

 apex. Lastly, in P. Radii (occurring on flower-heads of 

 Composite), with which the majority of species of 

 Peronospora agree, the apical opening has ceased to be 

 functional, and the hypha grows out laterally. In these 

 latter instances the sporangium has, in fact, become 

 a spore. Such asexual spores of Fungi, germinating 

 directly, bear the name of conidia. 



We will now return to our type Pythium. Zoospores 

 and conidia afford a rapid means of propagation so long 

 as a plentiful supply of victims, in the shape of young 

 seedlings, is forthcoming. Pythium is not, however, 

 altogether limited to a parasitic mode of life, for if host- 

 plants are wanting, it can live for some time as a 

 saprophyte on any decaying organic matter which may 

 happen to be at its disposal. Provision, however, has 

 to be made for bad times when food fails altogether, or 

 when there is not enough moisture for active vegetation 

 to go on. Such contingencies are provided against by 

 the formation of resting '-spores , which are the result of a 

 sexual process. 



b. Sexual 



The organs of sexual reproduction (oogonia and 

 antheridia) may be produced either inside the tissues 

 of the host-plant, or on hyphae which have grown out 



