Iv] Mildews 51 
off as conidia (see Fig. 17). Each cell of the hypha 
in its turn becomes the terminal one and forms a 
conidium. The formation of these gives rise to the 
powdery appearance. External conditions play a con- 
siderable part in their formation. In muggy weather 
they are produced in enormous numbers. Dampness 
is also required for their germination. This is why the 
disease is so prevalent under damp conditions. In 
bright sunny weather fewer erect hyphae and conse- 
quently fewer conidia are formed and conditions are 
less favourable to their germination. The conidia are 
oval in shape and collectively may be whitish grey or 
pink. On germinating they send out tubes which, if 
they happen to fall on the same kind of plant as the 
one on which they were formed, immediately send 
out haustoria into the host plant. The mycelium so 
produced resembles the parent mycelium. In this 
way the disease is spread rapidly from plant to 
plant. 
As the season advances the mycelium has a darker 
appearance, and here and there among the hyphae a 
number of dark bodies are formed. Under the micro- 
scope these appear as a number of dark brown spherical 
bodies attached to the mycelium on their under surface 
(see Fig. 18). By separating them from the mycelium 
they are seen to have a shiny appearance and polygonal 
markings covering their surface (Fig. 17). They are the 
“spore cases” of the fungus, and as they get older they 
become darker in colour. When ripe their walls are 
brittle. Put some on a slide under a coverslip and tap 
it; the spore cases will burst open, revealing a number of 
somewhat pear-shaped bodies which contain a varying 
number of spores inside them. The latter are known 
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