ALGAL AND FUNGAL PROTISTS 



35 



a basis for comparison with algae and with kindred genera 

 of primitive fungi. Facts mentioned below combine to 

 make this genus one of the most important for practical 

 study. 



Specimens of water-species can be obtained by the same 



a 



FIG. 6. PYTHIUM. a, seedling of cress, the arrow points to the part 

 invaded by the parasite ; 6, end of a hypha which has pierced the 

 wall between two host-cells ; c, conidia of Pythium debaryanum, 

 above, ripe terminal, below, early interstitial ; d, oogonia and an the - 

 ridia, above early stage, below, oosphere contracting, fertilisation 

 canal formed ; e, two stages of the germinating zoosporangium of 

 P. proliferum ; f, zoospores. Portions of Marshall Ward's illustra- 

 tions. 



means as Saprolegnia, a slice of raw potato makes a good 

 substratum. One sub-genus, Aphragmium, by its mode of 

 reproduction is very like a filamentous alga : without septa 

 even being formed, an unaltered portion of its mycelium is 

 transformed into zoospores, which escape at a hole in the 

 wall. The slender pythium-filaments are multinucleate. 



