io8 DISEASES OF TREES 



The stroma, which first produces paraphyses and later asci, 

 originates underneath this mycelial body, which is firmly 

 attached to the epidermal cells. 



The moister the weather, the more rapidly do the spores 

 ripen. They are disseminated only when a long spell of wet 

 weather has saturated the dead leaves with water and enabled 

 the paraphyses and spore-walls to swell by contact with the 

 water. This swelling leads to the rupturing of the leaf and 

 the formation of a longitudinal fissure, which immediately 



FIG. 51. Hysterium macrosporujn, showing a transverse section through a ripe 



ruptured stroma. 



recloses on the occurrence of dry weather or when the spores 

 have escaped (Fig. 51). 



HYSTERIUM NERVISEQUIUM 1 



The distribution of this disease is coterminous with that of 

 the silver fir, though I have found it really injurious only in the 

 Erzgebirge, where large woods of silver firs, even of advanced 

 age, had lost the majority of their leaves. Brownness of the 

 leaves is always observed for the first time from May to July, 

 and occurs on the two-year-old leaves which are entering their 

 third year. A few months after the leaves have turned brown 

 the spermogonia develop on their upper side, where two sinuously 

 corrugated black longitudinal ridges make their appearance 

 (right part of Fig. 53). 



1 R. Hartig, Wichtige Krankheiten, pp. 114 et seq. 



