HYMENOMYCETES. 423 



any sporophores and applying tar to the wound, while diseased 

 stems in the forest should be felled. Immediate artificial 

 closure of wounds in the wood is a very effective preventive 

 measure.^ 



The Hymenomycetes are divided into Tomentelleae, Exohasi- 

 diaceae, Hypochnaceae (included by Brefeld in the Tomentelleae), 

 Thelephoreae, Olavaricae, Hydneae, Polyporeae, and Agaricineae. 

 All contain parasitic species. 



EXOBASIDIACEAE. 



Exobasidium. 



The basidia are formed on the extremities of branches of 

 the mycelium, which break out through the cuticle of attacked 

 organs. The mycelium lives inside the host-plant, and induces 

 considerable malformation. The basidia emerge on the surface 

 of the host (similarly to the asci of the Exoasci), and from 

 each of the four sterigmata a single spore is given off. 



Exobasidium vaccinii Wor.^ (Britain and U.S. America). 

 This is the cause of a very common and conspicuous deformation 

 which affects the leaves, flowers, and shoots of Vaccinium Vitis- 

 Idaea (Fig. 256). Leaves, where affected, become thickened 

 and form irregular blisters vaulted towards the lower surface 

 of the leaf, so that the lower epidermis covers the convex 

 side and the upper epidermis lines the concavity. Chlorophyll 

 is absent in the swollen tissues, but where blisters are exposed 

 to direct light a bright red cell-sap is developed. Parts of 

 the leaf adjoining diseased spots may remain normal and 

 green. Flowers or their parts undergo similar malformation ; 

 twigs become more or less thickened and twisted, their chloro- 

 phyll disappears, and a reddish cell-sap is produced. On siich 

 diseased places spores are produced during the summer, after 

 which the poorly developed tissues dry up and wither. 



When this fungus is present in the young tissues of its 

 hosts, it exerts a very marked influence on their development. 

 The palisade cells of the leaf become enlarged, while their 

 chlorophyll almost wholly disappears, and is replaced by a red 



' Further details on this point have already been given. General part, p. 72. 

 ^Woronin, Verhand. d. naturfor. Ges., Freiburg, 1867 ; with 3 plates, 

 Brefeld, Schimmelpihe, viii., 18S9. Wakker, Pringsheim's Jahrbuch, 1892. 



