7 2 DISEASES OF TREES 



Whether this view will receive confirmation in the future 

 remains to be seen, but in the meantime its correctness is open 

 to grave doubts. In the first place, it has not yet been proved 

 that trees can take in organic food-substances by their roots ; and, 

 in the second, it has been established that trees are very well 

 nourished without the aid of Mycorhiza, and that, besides the 

 infested roots, there is always a very large proportion of roots 

 entirely free from fungoid growth. 



PYRENOMYCETES 



In the case of Pyrenomycetes the hymenium bearing the asci 

 usually lines the inner surface of roundish or flask-shaped 

 receptacles, called perithecia, which are distinguished from 

 the cleistocarps of the preceding by having an aperture at the 

 apex through which the spores escape. The numerous genera 

 may be divided into two groups, according as the perithecia 

 stand singly (simp.lices), or grouped in large numbers on a 

 common cushion, or sunk in a stroma (compositi). 



The following species, being noteworthy parasites, deserve 

 closer attention. 



TRICHOSPH/ERIA PARASITICA l 



This parasite is chiefly met with on the silver fir, though, 

 according to v. Tubeuf, it also occurs on the common spruce 

 and hemlock spruce. It is to be found wherever the silver fir 

 is indigenous. Its colourless perennial mycelium grows on the 

 under side of the branches, from which it spreads to the under 

 side of the leaves, knitting them firmly to the branches. On 

 this account the leaves, instead of falling off on dying, remain 

 attached to the branches (Fig. 18). 



On account of the mycelium being confined to the lower side 

 of the branch, most of the leaves that are met with on the upper 

 side survive during the first year at least (Fig. 18, a). The my- 

 celium encroaches on the new shoots as they are formed, and, 



1 R.Hartig, Ein neuer P arasit der Weisstanne^ Trichosphceria parasitical 

 Allgem. Forst- und Jagd-Zeitg.^ January 1884. 



