244 DISEASE IN I'LANTS. 



on the necrotic tissue of a wounded twig, and 

 having laboriously accumulated a vigorous my- 

 celium saprophytically, extend into other parts. In 

 many of these cases the dying-back of the twigs 

 is expedited owing to the mycelium invading the 

 medullary rays and wood vessels, and so obstruct- 

 ing the transpiration current. The much more 

 rapid spread of the hyphae up into the parts thus 

 killed sufficiently indicates the fundamentally 

 saprophytic character of such fungi. 



Dying-back in all its forms is a common result of 

 defoliation by insects, e.g: caterpillars, especially if 

 it occurs when the wood is depleted of reserve 

 materials, and thus cannot supply the auxiliary 

 buds and enable the twigs to clothe themselves 

 with a new flush of foliage, a common danger in 

 Conifers. 



Any form of defoliation e.g: excessive plucking 

 of tea and mulberry leaves, browsing of animals, 

 etc. exposes the twigs to the dangers of dying- 

 back, the accessory phenomena being similar to 

 those already described. 



Stag-heaiT. Old trees, though vigorous and in 

 full foliage throughout the crown generally, fre- 

 quently lose the power of bearing leaves on their 

 topmost branches and twigs, which stand out bare 

 and brown, and fancifully resemble the antlers of a 

 stag : hence the forester's name "stag-head." This 

 "top-dry" condition is frequently due to the re- 

 moval of litter, or to excessive draining, or to the 

 roots having gradually penetrated into unsuitable 

 soil. The consequence is that some dry summer 



