v] 



SPHAERIALES 



161 



Pleosporaceae 



The Pleosporaceae are saprophytes or in a few cases parasites, for the 

 most part on seed plants but in some cases on Pteridophyta, Bryophyta or 

 Lichens. The penthecia are immersed in the substratum, the ostiole only 

 projecting, but they may become more or less exposed by the rupture of the 

 covering tissues. The peridium is leathery or membranous. 



The genus Pleospora includes some 225 species, several of which occur 

 on grams and other grasses where they show biological specialization 

 Pleospora herbarum is a facultative parasite on the leaves of angiosperms 

 the penthecium is initiated by the division of a hypha into numerous short 

 cells from which branches grow out. The central cells, and later the basal 

 parts of the branches, divide in various directions till an irregular paren- 

 chymatous mass is formed. By further growth and division the mass 

 assumes a globular shape and the central cells become elongated and differ- 

 entiated as paraphyses. Later, asci appear, developing from the same cells 

 as the paraphyses and each produces eight muriform spores (fig. 120). 



Fig. 120. Pleospora sp.; germinating spores, x 1000. 



Multicellular conidia also develop on branched hyphae, the terminal cells 

 of which form the sterigmata. After the spore is shed the hypha may continue 

 to grow, a new sterigma being formed above the old one. The name Macrospo- 

 rimn parasiticum was formerly applied to the conidial stage of this species. 



The genus Venturia includes over fifty species, several of which are para- 

 sitic on living leaves; the perithecium is immersed and the large ostiole beset 

 with stiff hairs or bristles. The species grouped under Fusidadium among 

 the Hyphomycetes are in some cases conidial forms of this genus. The 

 conidia are two-celled, borne on short conidiophores arranged in groups; 

 F. dendriticum is the cause of scab or black-spot on apples, and F. Pyrinum 

 of a similar disease on pears. 



G.-V. i 1 



