288 PTTHITJM. 



some actively budding Yeast on a slab of plaster-of-Paris, 

 made by pouring the plaster mixed with water into a 

 greased vessel or on a piece of wood. (3) Spread some 

 active Yeast on a slice of Potato, and keep it under a bell- 

 glass. 



PYTHIUM DEBARYANUM. 



414. Material for Study. Sow seeds of Common 

 Cress (Lepidium sativum) thickly in a pot of wet sawdust 

 or loose soil, or on muslin stretched across a tumbler of 

 water, cover with a glass plate or bell-glass, and keep the 

 seedlings thoroughly wet and in a saturated atmosphere. 



Note that in a few days the seedlings become weakened 

 and fall over; a thick web of fungus-threads appears 

 binding the seedlings together ; and finally they become 

 completely decayed. Quite early they show a pale sickly 

 appearance, the hypocotyl becomes constricted and softened, 

 and bending occurs here. 



The fungus, which begins by attacking the live seedling 

 as a parasite, kills it and then thrives as a saprophyte on 

 the decaying tissues of the dead seedling; the spores of 

 Pythium are present, along with those of Bacteria and 

 countless Fungi, in the air, and these spores produce 

 threads which penetrate the hypocotyl and ramify in the 

 tissue of the seedling. 



415. Structure of Thallus and Gonidangia. 



Mount some of the infected seedlings in water, and note 



(1) That the tissues are disorganised and yellowish at 

 the base of the hypocotyl, where the seedling has collapsed 

 owing to the attack of the Fungus. 



(2) The colourless branching fungus-threads (hyphae) 

 running along the surface of the seedling, entering either 

 by a stoma or by boring through the epidermal cells. 



" (3) The hyphae running through the intercellular spaces 

 inside the seedling, or through the cells themselves. 



(4) The unseptate or coenocy tic structure of the hyphae, 

 i.e. the absence of cross-walls, as in Vaucheria. 



