GROUP I. THALLOPHYTA: FUNGI: ^CIDIOMYCETES. 303 



two sub-orders Pezizaceae and Helvellaceee. In the former the apothecium is 

 cup-shaped, the hymenium covering the concave surface, and is closed in the 

 early stages of its development ; in the latter the apothecium is borne on the 

 convex, smooth or reticulate surface of an erect stroma. 



The sub-order Pezizacese includes several families, the Phacidie, Pezizese, 

 Bulgarieas, etc. As representative may be mentioned Rhytisma Acerinum, the 

 mycelium of which infests the leaves 

 of the Maple, but the development 

 of the apothecium does not take 

 place until after the leaves have 

 fallen; and other similar forms 

 which inhabit the leaves of the 

 Silver Fir, Spruce, and other trees : 

 Ascobolus, which grows on dung : 

 the various species of Peziza, with 

 brightly coloured apothecia, growing 

 on rotting wood, etc.: Bulgaria, with Fm " 213. - Longitudinal section of the 

 . apothecium of Peziza convexula: ft the hy- 



a gelatinous apothecium, growing on menilim . (After Sachs.) 

 dead branches of the Oak. 



The sub-order Helvellacese includes the genera Morchella (the Morel], escu- 

 lent), Gyromitra, Helvella, etc. 



Sub-Class Y. ^ECIDIOMYCETES. This sub-class includes a con- 

 siderable number of parasitic plants known as Rusts and Smuts. 

 They are characterised by their remarkably complex life-history, 

 due to the polymorphism of what represents the gametophyte, 

 which presents two or more gonidia-bearing forms : and by the fact 

 that neither gonidia nor spores are developed in the interior of a 

 sporangium or gonidangium, but are formed by abstrictioii. A 

 sporophyte is indicated in one order of the sub-class, but not in 

 the other, and this constitutes the essential difference between 

 them ; it is indicated by the fructification which is termed an 

 JEcidium. Whilst, as a matter of fact, it is not certainly known 

 that the plants in question have sexual organs, and that the 

 aecidium is the product of a sexual process, there are some grounds 

 for regarding the aacidium in this sub-class as thehomologue of the 

 ascocarp in the Ascomycetes, and for the view that, in both sub- 

 classes, the aecidium and the ascocarp respectively represent the 

 sporophyte (p. 279). 



The sub-class is divisible into two orders : 



Order 1. Uredineas : have an ascidium-form, as a rule. 



Order 2. Ustilagineaa : never have an ascidium-form. 



Order I. Uredineae. This order comprises those parasites which are 

 generally known as Kusts, on account of the rusty appearance which they give 



