676 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



sort of cortex around the central spore which germinates as in Tilletia. Urocystis 

 ViolcB is common on the leaf-stalks and blades of Violets. 

 About 100 species of Tilletiaceae have been distinguished. 



Sub-class III. — Mtcomtcetes. 



Mycelium many-celled. Reproduction asexual, either by spores of limited num- 

 ber in asci, or by conidia of limited number on basidia. 



Alliance XVIII. — Ascomycetes. 



Parasitic (or saprophytic) Fungi, producing spores in special tubular sporangia, 

 the asci. These spores, termed ascospores, do not exceed 8 in number. In addition 

 to the asci there are subordinate conidial fructifications. 



Families: ExoasoaceoB, Perisporiacece, Pyrenomycetes, Discomycetes. 



As stated, this alliance is characterized by the possession of sporangial fructifi- 

 cations, consisting of tubular asci containing as a rule 8 ascospores. A number of 

 such asci are shown in fig. 388 ^ with sterile supporting hyphse, the paraphyses, 

 between them. In addition to these ordinary and typical ascus-bearing fructifi- 

 cations, secondary fructifications producing conidia or chlamydospores are largely 

 met with; consequently many of these Fungi appear under several forms in addition 

 to the ascus stage. The Ascomycetes are divided into families according to the 

 characters of the ascus-fruit. In the ExoascacecB the asci are borne freely and 

 exposed on the mycelium; in the other three families in special receptacles. In the 

 Perisporiacece the group of asci is contained in a nut-like or tuber- like body; in 

 the Pyrenomycetes the asci are produced in special pear-shaped excavations in a 

 solid tissue which open by a pore to the exterior; in the Discomycetes the receptacle 

 forms an open plate or cup, or sometimes an irregular body covered with the layer 

 of asci. 



Exoascacece. — This family comprises the parasitic genera Exoascus and Taph- 

 rina, the gall-like deformations caused by which were so fully described on pp. 524 

 and 527. The tissues of the host-plants are penetrated hy the mycelia of these 

 forms, and the asci are produced over the surface of the parts attacked, generally 

 bursting through the cuticle of the epidermis. Each ascus contains 8 spores, but in 

 many species many more than this number are often found; this is due to the fact 

 that the 8 original ascospores begin to bud whilst still within the ascus, producing 

 a large number of secondary spores (conidia). Exoascus has a perennial mycelium, 

 and to its species are due a large number of the " witches'-brooms " and other 

 hypertrophies. E. Pruni produces the "pocket-plum" (cf. p. 524); E. Alni-incance 

 the curiously altered Alder catkins represented in fig. 358 ^ (p. 523); E. Garpini the 

 birds'-nest-like witches'-brooms of the Hornbeam ; E. deformans the " curl " of 

 Peach-leaves. Taphrina is largely a leaf-parasite, and its mycelium is not peren- 

 nial. T. carnea causes blisters on the leaves of the Birch. 



