THE FUNGI 219 



appears often enough when it is not wanted. The 

 disease caused by it is well known to gardeners as the 

 " damping off " of seedlings. The stem of the seedling 

 when attacked by the Fungus soon tumbles over on to 

 the ground. This is because the outer tissues of the 

 stem, at the part where it gives way, have been so 

 much damaged by the parasite that the stem has not 

 the strength to stand upright. The fallen plants lose 

 their colour and soon completely rot away. 



1. STRUCTURE 



In its vegetative condition, Pythium consists of long, 

 fine, irregularly branched filaments or hyphce\ the 

 latter name is given to the filaments of Fungi in general. 

 These hyphae are for a long time without any trans- 

 verse walls ; they are in fact non-cellular, just like the 

 filaments of a Vaucheria. The inside of the hypha 

 is occupied by vacuolated protoplasm, in which very 

 numerous nuclei are embedded. Unlike Vaucheria, 

 however, Pythium has no chlorophyll and no plastids. 

 Neither is starch formed, either in this genus or in 

 any other Fungus. 



It is usual to speak of the whole vegetative body 

 or thallus of a Fungus as the mycelium. In Pythium, 

 then, the mycelium of each plant is made up of all 

 the hyphaa collectively, which have sprung from a single 

 spore. The mycelium of Pythium penetrates the tissues 

 of its victim or " host," and spends most of its vegetative 

 life within them. A hypha can make its way into the 

 stem either by way of a stoma or by boring directly 

 through the cuticle ; it goes on growing and branching 

 inside the host-plant, where it is not confined to the 



