INJURIES INDUCED BY PLANTS 89 



NECTRIA 



The genus Nectria contains a number of parasitic fungi which 

 produce their perithecia which are usually red, and grouped in 

 considerable numbers on the surface of a pseudo-parenchymatous 

 wart-like stroma. Before the perithecia make their appearance, 

 this same stroma serves for the production of numerous gonidia. 

 The gonidia-bearing stroma was formerly referred to a special 

 genus called Tuber cularia. 



The following three species belonging to this genus are faculta- 

 tive parasites, which, like so many other parasites, can also live 

 as saprophytes. 



NECTRIA CUCURBITULA 1 



Like all the Nectrias, N. Cucurbitula is one of those parasites 

 which, as a rule, can gain access to the interior of a host-plant 

 only through a pre-existing wound. Here the host-plant is 

 usually the spruce, in rarer instances the silver fir, Scotch pine, 

 &c. The means of entrance which the parasite utilizes in the 

 forest are chiefly the injured spots due to Grapholitha pactolana, 

 Fig. 37, though, less frequently, it also enters through the abra- 

 sions caused by hail, or the crack at the base of a branch whose 

 bark in the upper angle has been slightly torn by the depression 

 due to an accumulation of snow. 



The germinating ascospores or gonidia push their germ- 

 tubes into the tissues of the cortex, and the ramifying mycelium 

 ultimately develops most luxuriantly in the sieve-tubes of the 

 soft bast, Fig. 38 b, or in the intercellular spaces between them, 

 Fig. 38 c. The mycelium is met with in bast tissues that are 

 apparently perfectly sound and fresh. The brown colour does 

 not appear in the tissues for some time afterwards. The 

 fungus would appear to make progress, for the most part, only 

 when growth is at a stand-still in the cortical tissues. It 

 generally ceases to advance when the plant and its cambium 

 awake to renewed activity. From this we must assume that 

 the power of resistance of the living tissues of the host-plant is 



1 R. Hartig, Untersuchungen, I. p. 88. 



