UREDINEAE AND HYMENOMYCETES 523 



In the Hymenomycetes, the basidia arise upon a fruit-body of 

 greater or less complexity and they lie with their axes parallel to 

 one another in a compact layer called the hymenium which covers 

 part of the outer surface of the fruit-body. Each basidium arises 

 from a sterile subhymenial cell from which it is cut off at the 

 moment of its formation ; and, soon after its formation, under 

 normal conditions of temperature and moisture, it produces and 

 discharges its basidiospores. The basidium cannot be considered 

 as a structure especially adapted to undergo a prolonged resting 

 period : its cell-wall is very thin, it easily loses water, and, except 

 for a number of species where the fruit-bodies usually grow on 

 wood and where every cell of the fruit-body can survive desiccation, 

 dies when it is dried. For instance, desiccation of the gills of a 

 Mushroom, a Tricholoma, a Psathyra, or a Coprinus is fatal to 

 every basidium which the gills contain. Moreover, a basidium does 

 not become independent of the subhymenial cell from which it is 

 derived but, until it has discharged its basidiospores, draws upon it 

 for its supplies of moisture. It would be hopeless to try to isolate 

 a basidium from the hymenium of one of the Hymenomycetes and 

 expect it to continue to produce its sterigmata and spores. 



In the Uredineae, on the other hand, there is no fruit-body 

 comparable to that which occurs in the Hymenomycetes ; and 

 such a fruit-body is unnecessary on account of the fact that, the 

 Uredineae are parasites : the stems, leaves, or inflorescences of their 

 hosts constitute admirable substrata upon the surfaces of which 

 the basidia can be produced. The tissues of the host-plants take 

 the place of the fleshy parts of hymenomycetous fruit-bodies, in 

 that they supply the food and water necessary for the development 

 of the basidia ; whilst the position of the infected leaves, stems, 

 etc., above the ground, is such that the basidiospores, when liber- 

 ated, escape into the air in such a manner that they can readily 

 be carried off by the wind. A structure like the stipe which 

 characterises the fruit-bodies of so many terrestrial Hymeno- 

 mycetes, and the chief function of which is to place the hymenium in 

 such a position that when the spores are liberated from the basidia 

 they will fall into an open space where the wind can carry them 

 away, is therefore rendered unnecessary in the Uredineae. 



