BASIDIOMYCETES 407 



the root tips of Cormophyta. These sclerotic, thread-like and apically 

 growing structures are called rhizomorphs. 



While all these threads with marked growth in length, especially in 

 their development as rhizomorphs, can penetrate far over unfavorable 

 areas, the tertiary mycelium of the sclerotia and fructifications (sporo- 

 phores) is characterized by very small power of growth and particularly 

 by the lack of a definite growth in length. The sclerotia result directly 

 from the secondary mycelium ; in structure they are similar to the rhizo- 

 morphs, consisting of a compact pseudoparenchymatous rind and a core 

 formed of laminated cells. Occasionally they are formed in large numbers, 

 reach only a small size and are called bulbils. When brought into 

 favorable surroundings, they usually develop to secondary mycelia, seldom 

 to fructifications. 



In contrast to the Ascomycetes, basidial fructifications arise, as the 

 sclerotia, from a tangle of secondary hyphae, seldom indirectly from 

 sclerotia, rhizomorphs or mycelial threads. In the latter case, their 

 tissues develop by degeneration from the tertiary tissue. In contrast to 

 the simple Ascomycetes, their appearance does not coincide with plasmo- 

 gamy, for in Basidiomycetes this is shifted into the vegetative mycelium 

 and the hyphae which form the fructifications have been binucleate for 

 considerable time. Fructifications in nature apparently are a complex of 

 numerous loosely intertwined, diploid individuals which have developed 

 from different spores and have copulated independently of one another a 

 long time before the formation of the fructification. In some species, 

 as in Coprinus nycthemerus, Armillaria mellea, Schizophyllum commune 

 Panaeolus campanulatus and Anellaria separata (Kniep, 1911, 1913, 1919; 

 Vandendries, 1923), the fructifications occasionally may develop parthe- 

 nogenetically from the uninucleate hyphae but they certainly develop 

 much later than normal fructifications. Purely physiological fac- 

 tors such as humidity, light relations, nutrition, etc., determine the 

 exact moment for the beginning of the fructification (Wakefield, 1909). 



In many terrestrial species in which the mycelium grows centrifugally, 

 the fructifications appear in concentric rings often called fairy rings; these 

 increase each year in diameter, e.g., in eastern Colorado, U.S. A., 12 cm. per 

 year with 60 cm. in very favorable years, and in unfavorable years none 

 at all (Shantz and Piemiesel, 1917). As the mycelium takes a part of its 

 organic substance from the ground, and returns it later in concentrated 

 form with the decomposition of the intertwined hyphae within a narrow 

 circular zone, they bring about definite ecological successions. In 

 Lycoperdon, Marasmius and Calvatia the growth is locally stimulated by 

 generous nitrogenous fertilization, in others, e.g., in Psalliota tabularis, 

 the plants are damaged or entirely killed, for some unknown reason. 



The fructifications are generally fibrous or gelatinous; at other times, 

 because of the thickening of the hyphal walls, they are firm; in the higher 



