10 SOME DISEASES OF CEREALS. 
In fully infected leaves the mycelium is collected chiefly 
between the cells ofthe mesophyll adjoining the bundles and also in 
the inner layers between these (plate ITI, fig. 3). When sporangium 
formation is about to begin, tufts of mycelium reach the air 
chambers of the stomata, which are, as in many grasses, arranged 
in parallel, longitudinal rows, one on each side of each vein. 
Sporangium formation is accompanied by great assimilative 
activity of the fungus, which so increases its demands on the 
living cells of the host, as to cause the latter to collapse and 
finally die. Prior to this, the chlorophyll of the assimilating cells 
is wholly or in part destroyed, and starch is noticeably absent in 
the cells which harbour the haustoria of the parasite. Hence 
the pale streaks, which are the first indication that young leaves 
are affected, are visible before any conidiophores are extruded, 
and these streaks only turn brown, from death of the cells, after 
sporangium formation has reached its maximum. 
The effects of the fungus on the tissues are evident both in 
the mesophyll and the bundles of the leaf. The former undergoes 
hypertrophy (** hyperplasie,’’ Kiister, 1903), the number of layers 
being increased without any modification in the structure of 
the cells. These remain always large, thin-walled cells, with 
small air spaces in the angles between adjacent cells. In the 
sheath, where a hypoderm of a single layer is often demarcated 
from the mesophyll on the upper surface, this may be increased 
to two or three layers. In the blade there is no hypoderm, and 
the increase takes place in the ordinary mesophyll layers, but 
is never considerable and is sometimes absent. On the whole, 
it may be said that the fungus occurs in greatest quantity in 
the blade of the ordinary leaves, and in the sheath of those 
produced by median prolification, and the hypertrophy produced 
is most evident in these parts. 
In the bundles, the number and size of both the xylem and 
phloem elements are increased. In normal bundles of moderate 
size, the xylem consists of a central spiral vessel, with a second 
(annular) vessel, or an air space, inside it, and two large lateral 
vessels, one on each side, The phloem consists of a group of 
