THALLOPHYTA 



461 



Order 5. Exobasidiineae 



No distinctive fructifications are formed, and the basidia spring in irregular 

 groups directly from the mycelium. They bear four spores on slender sterigmata. 

 Exobasidium Vaccinii may be taken as a type of this form. The mycelium of 

 this fungus, which is widely spread in Europe, is parasitic on the Ericaceae, 

 especially on species of Vacdnium ; it causes hypertrophy of the infected parts. 

 The basidia are formed in groups under the epidermis, which they finally rupture 

 (Fig. 409). In this genus, as in many others, accessory fructifications are developed, 

 and spindle-shaped conidia 

 are abstricted from the my- 

 celium on the surface of the 

 host plant, before the forma- 

 tion of the basidia. 



Order 6. Hymenomycetes ( 70 ) 



The basidia are undivided, 

 and bear four spores at the 

 apices of slender sterigmata 

 (Fig. 410 sp). They are 

 produced on fructifications, 

 which bear definite hymenial 

 layers, composed, in addition 

 to the basidia, of paraphyses 

 (Fig. 410 p), and also of 

 sterile cystidia (c) or club- 

 shaped tubes characterised 



by their larger diameter and 



, ,, . , , ,, FIG. 410. Russula rubra. Portion of the hymemum. sh, 

 more strongly thickened wall. Sub . hymenial layer : 5, basid ia ; s, sterigmata ; sp, spores ; , 



The four spores are pro- 

 jected from the sterigraata 

 by means of the osmotic pressure of the basidium to a distance of about ^ mm. ; 

 they readily adhere to any surface. The paraphyses by separating the basidia 

 facilitate the free shedding of the spores. The cystidia, according to KNOLL, are 

 organs for secreting water and mucilage. They may have other functions in 

 particular cases ; thus in Coprinus they hold apart the gills and ensure the free fall 

 of the spores ( 82 ). 



In the Hymenomycetes, as in the most nearly related orders, special sexual 

 organs are wanting and the basidia correspond to the asci of the Ascomycetes, and 

 like these have, to begin with, two nuclei which then fuse. The question thus 

 arises in what way the binucleate condition of the young basidium is brought 

 about and what homologies exist with Ascomycetes in the course of development ( 83 ). 



More recent investigations, especially those of KNIEP, have shown that in many 

 Hymenomycetes a mycelium consisting of uninucleate cells is developed from the 

 uni- or bi-nucleate basidiospores ; that sooner or later before the formation of the 

 fructification the binucleate condition is attained ; that the pairs of nuclei show 

 conjugate division ; and that the binucleate condition is associated with the 

 peculiar formation of clamp connections until the formation of the basidia. The 

 clamp connections arise in the same way on the vegetative hyphae composed of 

 elongated cells and on the shorter and stouter hyphae from which the hymenium is 

 formed. In both cases a short protrusion forms about the middle of a terminal cell 

 of a hypha (Fig. 411, 1). One of the two nuclei passes into the protrusion 



paraphyses ; c, a cystidium. (After STRASBURGER. x 540.) 



