A DISEASE OF THE BLACK LOCUST. 25 
ture of the timber. Wood which has large vessels, prom- 
inent medullary rays, resin channels, or the wood elements 
of which are large-lumened and thin-walled, will be pene- 
trated throughout its entire mass more readily than wood 
where those natural channels are absent, or which has 
short thick-walled wood elements. As a rule it was found 
that the course of hyphae in all the timber so far examined 
is first through the medullary rays and vessels, from which 
points individual hyphae penetrate the woodcells adjoin- 
ing; in other words it appears that growth directly 
through a solid mass of wood rarely takes place, and when 
it does so it is a very slow process. 
The wood elements of the locust are short, thick-walled 
and resistant, and are penetrated by the hyphae very 
slowly. The medullary rays on the other hand are large 
and continuous, and are composed of small comparatively 
thin-walled parenchymatous cells. These are rapidly in- 
vaded and destroyed by the hyphae, and it is through them 
that the mycelium extends outward from the point of 
infection. This accounts for the peculiar sheets. The 
fungus advances through the large medullary rays, and 
from them the hyphae grow laterally and vertically into 
the surrounding vessels and wood cells. Progress laterally 
is made difficult, because the hyphae have to bore their 
way through the solid walls of the wood fibres and it, 
therefore, goes on slowly. Progress longitudinally is more 
rapid, because here the large ducts permit of a rapid advance 
up and down, and even the woodcells are more easily 
reached. It will readily be comprehended how a spread of 
this kind through wood of the nature of the locust must 
tend to form sheets of the kind described. One finds sim- 
ilar cases among some of the oaks, but not to such a 
marked extent, for there the woodcells are not so resistant 
nor so closely packed together. 
A piece of wood taken from the vicinity of one of the 
sheets will show changes as represented in Plate 3. The 
