INJURIES INDUCED BY PLANTS 105 



from which one or two much-septated germ-tubes are deve- 

 loped laterally or terminally. The perithecia, which afterwards 

 predominate, appear at the same time. As such they remain 

 through the winter and the following spring. They are 

 spherical in shape, and measure 0*2 mm. in diameter, and 

 contain paraphyses and asci, the latter having eight bicellular 

 spores. 



The disease may be combated by lopping off the lower 

 branches of infected trees. 



The gonidium form of this fungus is Fusicladium Tremulce, 

 which develops on leaves that have been infected in the previous 

 spring by the ascospores. 



DISCOMYCETES * 



The essential difference between the Discomycetes and 

 Pyrenomycetes is that the asci are not formed on the internal 

 wall of a closed spherical or flask-shaped organ (perithecium), 

 but on the surface of an open saucer-shaped fructification 

 (apothecium). Before the ripening of the spores the asci are, 

 at most, protected by a covering which does not belong to 

 the fructification itself, but is partly formed out of the epidermal 

 layers of the host-plant. 



The Discomycetes are arranged into several sub-families, 

 amongst which the Phacidecz are to be distinguished by the 

 hymenial layer, originating not on the surface of the fungus-body 

 but in its interior, where it remains covered, temporarily or 

 permanently, by the fungal tissues. 



Special mention may be made of the genera Rhytisma and 

 Hysterium, which belong to this group. 



RHYTISMA ACERINUM^ 



This fungus causes one of the best known of the blotch- 

 diseases of the Maples. Acer platanoides suffers especially from 

 this parasite, A. pseudoplatanus and A. campestre in a less degree. 

 On the leaves in July we first of all observe round yellow 



1 Cornu, Compt. rend., Ixxxvii. (1878), p. 178. 



* [The reader may be referred to Phillips's British Discomycetes (Kegan 

 Paul & Co., 1887) for an account of our native forms. ED.] 

 f [Very common in this country. ED.] 



