CHAPTER V. — COMPARATIVE REVIEW. — ASCOMFCETES. 11$ 



b. A richly vegetating primary mycelium or thallus proceeds from the ascospore, 

 and its complete development closes with the formation of sporocarps, but as a matter of 

 fact it always first produces gonidiophores with gonidia. The germinating gonidia again 

 give rise in all cases to a mycelium or thallus of the same qualities and capabilities as 

 those which sprang from the ascospore. Here therefore the formation of gonidia 

 is not necessary to the development, but it is, as a matter of fact, an invariable 

 occurrence. It precedes the formation of sporocarps in the history of the individual, 

 and the gonidiophores were therefore often termed the precursors of the sporocarps. 



The formation of gonidia is usually extremely copious in this third category, 

 often much more copious than that of the sporocarps, and then generally owing to 

 external causes which can be demonstrated ; it may be the only mode in which 

 the species is propagated during many successive generations, while sporocarps 

 appear exceptionally and under special conditions. Some species have more than 

 one kind of gonidia, which may then be distinguished according to size as micro- 

 gonidia, megalogo?iidia, macrogonidia, or by special names in particular cases according 

 to other characters. 



Again, the gonidia are formed in certain cases on the free surface of the thallus 

 on single hyphae or in crowded hymenia ; or else in receptacles resembling perithecia. 

 These latter have been termed by Tulasne pycnidia, and the spores or gonidia formed 

 in them stylospores — not very happy expressions ; the first, however, may be retained 

 here, the latter replaced by the words pycnospores or pyciiogonidia. 



All known gonidia in the Ascomycetes are acrogenously abjointed after one 

 or other of the forms described in section XVI, and none are swarm- cells. 



Examples of species which have been fully examined. The germinating 

 ascospore in the Erysipheae (Fig. 107) puts out a short germ-tube, which on a 

 suitable substratum, namely the living epidermis of certain Phanerogams, sends first 

 of all a haustorium (Fig. 6) into a cell of the epidermis, and then developes into the 

 filamentous branched thallus which spreads over the whole surface. Short erect 

 branches of this thallus then serially and successively abjoint large colourless 

 cylindrico-ellipsoid gonidia, and each of these gonidia yields in germination under 

 favourable conditions the same product as the ascospore. Every thallus which 

 proceeds from these germinations ends, when it has reached its full development, 

 with the production of archicarps and antheridia, that is, of sporocarps. But it need not 

 always arrive at this conclusion, but may only form gonidia and propagate itself by means 

 of them through an unlimited number of generations. This imperfect development 

 may usually be traced to obvious external causes, such as climatic conditions or the 

 absence of the nutrient substance required for perfect development, that is, the proper 

 species of Phanerogam. The Erysiphe of the grape-vine is the best example of the 

 group 1 . From the circumstances attending its first appearance and its diffusion in 

 Europe, it may be safely assumed that it suddenly migrated, and was transferred to 

 our vines from some other species of Phanerogam. It most probably came from 

 America. In spite of its destructive diffusion over the whole of wine-growing Europe 

 the most careful examination has never detected any sign of a sporocarp ; the 

 invasion was entirely carried out by vast numbers of gonidia, the shape of which 

 procured the plant the name of Oidium (O. Tuckeri, Brk.). The sporocarps are 

 probably found in N. America on native species of Vitis and described as Erysiphe 

 (Uncinula) spiralis, Brk. and Curt., but this is not certain. 



1 Beitr. z. Morph. u. Phys. d. Pike, Ill, p. so. 

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