248 SMUTS (USTILAGINE^E) MUSHROOM (AGARICUS) 



whilst in others (e.g. Puccinia malvacearum) only teleutospores 

 are known. 1 



Some of the Rusts exhibit very extreme specialisation in 

 relation to definite host-plants, possessing biologic strains which 

 can only develop on one particular species. Others, however, 

 can attack a variety of related hosts, and, in the continuance 

 of a disease of cultivated crops, wild plants may often play an 

 important part in bridging the interval of a rotation. 



The Smuts (Ustilaginea) are characterised by the sooty black 

 mass of spores which are formed by the breaking up of the hyphae 

 into unicellular chlamydospores (cf. p. 238) with a thick pig- 

 mented wall. They occur extensively as diseases of various 

 Cereals (e.g. Oats, Wheat, Maize), infesting either the leaves 

 (Fig. 134, G) or the ovaries. 



The common Mushroom (Agaricus campestris) affords a typical 

 example of the Basidiomycetes proper. The mycelium, which 

 inhabits soil rich in humus, and is present in considerable amount 

 in so-called Mushroom spawn, is composed of binucleate cells. 

 The hyphae, as in many other Basidiomycetes, tend to be inter- 

 woven in bundles, so that the mycelium appears thicker and 

 coarser than in other Fungi. The overground edible portion is 

 the reproductive body which first appears on the mycelium as 

 a knob-like swelling (Fig. 132, b) composed of densely interwoven 

 hyphae, but later, as it gradually enlarges, broadens out at the 

 top (Fig. 132, c, a). In the mature condition it consists of a 

 stalk and an umbrella-shaped cap (cf. also Figs. 137, 138), with 

 a large number of radiating plates or gills, which bear the 

 hymenium, protruding from the under surface. A little way below 

 the cap the stalk is surrounded by a membranous ring of broken 

 tissue (the annulus, Fig. 138) which, before the expansion of the 

 cap, extended continuously from the edge of the latter to the stalk, 

 thus constituting a protection for the developing gills (Fig. 132, a). 

 In a vertical section through the cap (Fig. 132, a, e) the middle 

 of each gill (d, /) is seen to consist of longitudinally arranged 

 hyphae. These are composed of rather large cells (t.), and diverge 



1 Other well-known and widespread Rusts are those occurring on the 

 Hollyhock (Puccinia malvacearum}, on the Wood Anemone (P. anemones), 

 and that causing purple spots on the leaves of the Blackberry (Phragmidium 

 bulbosum). 



