PYTHIUM 101 



is the cause of the trouble. Minute spores of this 

 parasitic fungus germinate under favourable conditions, 

 and the fine, colourless, branched filaments which grow 

 from the spores attack the seedlings and live at their 

 expense. These filaments are called hyphce, and the 

 complete thallus consisting of hyphse arising from a 

 single spore, in any fungus, is termed the mycelium. 

 The mycelium, then, is really a vegetative thallus 

 primarily engaged in the business of nutrition. 



The reader will remember that in our study of the 

 Alga Vaucheria (p. 66) we observed the non-cellular 

 structure of its filaments; microscopic examination 

 of the hyphse of Pythium reveals the fact that septa are 

 either completely absent or extremely rare ; so that they 

 resemble Vaucheria in this respect. Further resem- 

 blance is found in the fact that the hyphse, although 

 non-cellular, contain protoplasm in which very numerous 

 nuclei occur. In fact, we might almost say that the 

 mycelium of Pythium is like the thallus of Vaucheria 

 in all respects with the one great exception it contains 

 no chloroplasts. 



The spore of Pythium germinates outside the host- 

 plant, and the hyphse run along the stem. They are, 

 however, not satisfied with mere attachment; they 

 are in quest of food, hence they force a passage into the 

 tissues of their helpless host and absorb the products 

 of its activity. Entrance may be gained through a 

 stoma (p. 21), or a hole is bored through the cuticle. 

 Once admission is secured the hyphae grow rapidly 

 inside the plant, penetrating into its cells, holding high 

 revel all the while, and ultimately traversing the whole 

 plant, whose death - warrant was signed at the first 



