310 PART 111. THE CLASSIFICATION OF PLANTS. 



thin- walled gonidia of the Ustilagineae (when present) correspond to the uredo- 

 gonidia of the Uredineas. The gametophyte of the Ustilagineae thus presents 

 very much the same polymorphism as that of the Uredineae ; but there is 

 nothing in the former order to correspond to the ascidium of the latter. 



The affinities of the Ustilagineae are of great interest. On the one hand they 

 are clearly allied, as shown in their life-history, to the Uredineae, and (as is 

 explained on p. 314) with the Basidiomycetes. On the other hand they are 

 allied to the Chytridiaceae, and less closely, to the Entomophthoraceae. They 

 differ from the Chytridiaceae, however, in that their mycelium is septate, and 

 in that the gonidia are produced by abstriction, and are not motile. Proto- 

 myces, however, closely resembles some forms included in the Chytridiaceas 

 (Physoderma), in that it produces its sporidia in the interior of the resting- 

 gonidium, and in that the sporidia, like the gonidia of Physoderma, are non- 

 motile ; but Protomyces has a septate mycelium, whilst Physoderma has not. 



The most important and the most common species are Ustilago Carbo, which 

 especially attacks Oats, but other Cereals and Grasses as well : U. Maidis, 

 which produces large tumours in the Maize, filled with resting-gonidia : 

 Urocystis occulta, which fructifies in the leaves and haulms of the Eye : Tilletia 

 Caries, the Smut of Wheat ; this is dangerous because the grains filled with 

 resting-gonidia remain closed, and are therefore harvested with the sound ones. 

 Many other species and genera infest wild plants. 



Sub-Class VL BASIDIOMYCETES. This sub-class includes 

 a large number of plants, both saprophytes and parasites, the 

 fructifications of which are well-known as Mushrooms, Toad- 

 stools, and Puff-Balis ; they are the most highly organised of the 

 Fungi. 



The body is a branched septate mycelium, growing in the sub- 

 stratum, and bearing the reproductive organs which come to the 

 surface. 



The reproductive organs are gonidiophores of two kinds, com- 

 pound and simple. Of these the compound gonidiophore is uni- 

 versal, and is characteristic of the sub-class; it constitutes the 

 fructification commonly known as a Mushroom, a Toadstool, etc. 

 The structure of the compound gonidiophore may be illustrated by 

 reference to the common mushroom (Agaricus campestris) . It con- 

 sists of a stalk, termed the stipe, bearing at its apex a large 

 circular, somewhat umbrella-shaped expansion, the pileus. On 

 the underside of the pileus are a number of radiating plates of 

 tissue, the lamellce (Fig. 219), covered with the gonidia-bearing 

 layer of cells, the liymenial layer or hymenium. The lamellae 

 collectively constitute the hymenophore. Towards the upper end 

 of the stipe is a ring of tissue, the annulus, the torn remains of a 



