Iv] Mildews 49 
In the first stages of the disease the leaves are 
covered with what looks like a dense spider’s web. 
On examination this is found to consist of a network 
of hyphae or mycelium ramifying on the surface of the 
leaves and very similar to the mycelium obtained on 
bread cultures. The hyphae are divided by cross 
walls and so consist of long chains of cells. They 
branch irregularly and cross each other to form the 
dense network. Sections of the mildewed leaf show that 
the mycelium is not present inside (a piece may be 
dragged across the section on cutting and appear to 
be running in the interior of the leaf). It lies very close 
to the surface and is fixed to the leaf at certain spots; 
sections show how this fixture is brought about. At 
intervals along the mycelium short swollen tubes are 
given off and penetrate the surface layer. These 
swollen tubes are known as haustoria. This fungus 
differs from those already studied in having its myce- 
lium entirely outside the plant which it is attacking. 
In the case of the other parasites the mycelium is 
entirely inside the plant and the only portions visible 
are the fructifications. This feature of having the 
mycelium outside the plant is characteristic of the 
“true mildews.” The haustoria which it sends into 
the plant serve as a means of attachment and also of 
absorbing the food supply. In favourable weather the 
mycelium spreads rapidly over the whole leaf. After 
a time a powdery substance is produced. This powder 
consists of the conidia. The hyphae which produce 
them differ from the ordinary mycelium in growing 
out at right angles to the surface of the leaf instead of 
parallel to it. These hyphae are divided up by cross 
walls and the cells at the end of each become rounded 
P.F. 4 
