104 CLASSIFICATION OF THE MICRO-ORGANISMS. 



by transverse divisions ; and the threads are almost always 

 branched, either by the formation of a branch at some 

 part or other of a segment, or by the terminal cell 

 becoming divided dichotomously during growth. The 

 group of hyphae, whether they are present in small 

 numbers or quite single, or whether they are united in 

 masses, is termed the thallus of the fungus. 



Varieties of In the thallus we have to distinguish mycelium, and 

 the mycelium. later fruit . bearing hyphaa. Before the development of 

 the fruit-bearing hyphae the mycelium is identical with 

 the thallus, which signifies the more or less diifused 

 and branched hyphae which have grown on an organic 

 substratum. As a rule by uniform spread of the 

 threads in all directions and by continued branching a 

 flocculent mycelium is produced. At times membranous 

 parenchymatous layers or fibrous bands are formed by 

 the close union of numerous hyphae. Under certain 

 circumstances the mycelium of many fungi assumes the 

 form of the so-called sclerotia, tuberous fleshy bodies 

 which develop secondarily from an ordinary mycelium. 

 In the sclerotium we can distinguish a cortical and a 

 medullary substance, the latter consisting of interwoven 

 hyphae, the former of the terminal cells of the hypha) 

 firmly bound together and surrounded by dark mem- 

 branes. The sclerotium must be regarded as a resting 

 form, from which a development of fluid-bearing hyphae 

 only occurs after a long time, and when the surround- 

 ings are constantly moist. 



The hyphae of the mycelium penetrate with great 

 energy into the nutritive substratum. In the case of 

 dead portions of plants the hyphae can bore through 

 the cell membrane, the molecules of the membrane in 

 contact with them being broken up. But even in the 

 case of living plants the parasitic fungi not only spread 

 on the surface, but the hyphae grow in between the cells 

 of the plants and send short projections, so-called 

 haustoria, into the interior of the cells ; or they pene- 

 trate through the cell walls, as in the case of dead plants. 

 In like manner animal membranes do not offer any 

 marked resistance to the penetration of the growing 



