FUNGI 



309 



Differentiation of the Thallus. 



It is usually possible to draw a fairly sharp distinction between the 

 vegetative part of the thallus, the mycele, and that portion specialised 

 for the bearing of propagative bodies, the sporophore. In reduced 

 forms, however, such distinction will be seen to be arbitrary, since 

 propagative cells are formed in such cases from otherwise undifferen- 

 tiated hyphal cells. The mycele, which serves the same physiological 

 purposes as roots in point of the absorption of nutriment and of attach- 

 ment, occurs in a variety of forms. The simplest and the typical form 

 is a loose filamentous condition, spreading on or in the substratum, and 

 sometimes provided with 

 special branches termed 

 Aaustoria, which pene- 

 trate cells of living 

 plants and act as special 

 organs of nutrition and 

 of attachment. Such are 

 the haustoria of Perono- 

 sporcEe and Erysipheae, 

 &c. A denser condition 

 of mycele is represented 

 by a membranous state 

 to be found in forms 

 vegetating on the sur- 

 face of fluids and in fungi 

 ■which attack wood, such 

 as dry-rot (Merulius 

 lacrymans, Fr.) and very 

 many others. Strands 

 of mycele are formed frequently in the Gasteromycetes and other 

 fungi possessing compound sporophores. The strand-raycele of Aga- 

 ricus melleus (L.), formerly known as Rhizomorpha, possesses a 

 highly-developed structure, in which a cortical and a medullary region 

 may be distinguished. Most compact of all forms of mycele are those 

 known as sderotes — densely compacted tuberous bodies. These con- 

 sist of a central medullary tissue composed of very compact uniform 

 hyphje, sometimes pseudo-parenchymatous, enclosed by one or more 

 layers of cortical cells with sclerosed walls. They are resting states of 

 mycele, and act as stores of reserve material. With sclerotes may be 

 classed such sclerotioid bodies as Pachyma and Mylitta, the develop- 



FlG. 269. — Intercellular mycelial hyphee {m), with haustoria 

 penetrating into cells (a), A, of Cystopits candidiis L^v. ; B^ of 

 Peronospora. cctloikeca de By. ( x 390). (After de Bary.) 



