INJURIES INDUCED BY PLANTS 107 



region of the leaf that is occupied by the mycelium : this in 

 the former case is a black blotch, and in the latter consists of 

 black spots on a green ground. In autumn, when the leaves are 

 quite yellow, the green colour of the chlorophyll persists in the 

 blotches for some considerable time. 



RHYTISMA SALICINUM 



Black blotches similar to those produced by R. acerinum often 

 occur on Salix purpurea, nigricans, Caprea, aurita, &c. These 

 are caused by Rhytisma salicinum, but are of relatively minor 

 importance. 



HYSTERIUM (HYPODERMA) 



The genus Hysterium possesses black elliptical to linear 

 fructifications, which project from the leaves as black, lustrous, 

 wart-like bodies. 



The spores are linear, their walls being externally 

 mucilaginous and swollen. In the case of the three following 

 species the germ-tube probably always enters by a stoma. The 

 mycelium spreads between the cells in the parenchyma of the 

 leaves of conifers, which consequently become brown and die. 

 Should the disease attack a leaf near the base at a time when 

 the upper parts are still healthy and capable of assimilating 

 under the influence of light, and should the transportation of the 

 products of assimilation from the leaf be prevented by the death 

 of the elements of the bast, the plastic substances collect in the 

 form of starch-granules in such large quantities as to completely 

 fill up the leaf. 



The tissues of the leaves, which are at first pale green, after- 

 wards become brown, and frequently the fructifications do not 

 develop in them for more than a year. The ascogenous 

 fructifications are often preceded by spermogonia, which in the 

 case of the silver fir (Fig. 53) are arranged on the upper side 

 of the leaf in two sinuous longitudinal ridges, whereas the 

 apothecia which produce the ascospores are united on the under 

 side of the leaf to form a single similar ridge. Both originate 

 by the mycelium penetrating into and rupturing the epidermal 

 cells. It then develops luxuriantly and forms a lenticular 

 fungus-body, which afterwards becomes deep brown in colour. 



