OBSERVATION OF COMMON FUNGI. 
217 
This is the way this and many other molds and other fungi spread, 
i. e., by the formation and distribution of spores. 
The Downy Mildews. 
These are parasitic on many cultivated and wild plants. That 
is, the mycelium grows in the tissues of the plant (the host) and 
uses some of its substance for food, thus causing a disease from 
which the host suffers or dies. Thus the powdery miidew of the 
grape {Plasmopara viticola ), the onion mildew, the potato blight, 
etc., are diseases of these plants caused by different species of 
downy mildew. The mycelium grows for example in the tissue 
of the leaves, forming irregular yellowish spots on grape leaves, 
and often giving a pale yellowish tint to onion leaves. 
Examine with a hand lens the under surface of these spots on a 
grape leaf. If this fungus is present and fruiting the spot will 
be seen to be covered with a delicate downy growth formed of 
branches which come from the mycelium inside of the leaf. 
These are the fruiting branches (or conidiophores) which bear 
oval spores or conidia. These conidia are the summer spores 
and are carried by the wind to other leaves. Here in dew or rain 
they germinate by sending out a delicate thread-like growth which 
enters by a stoma into the leaf and forms more mycelium and 
more disease. In the late summer or autumn the internal myce¬ 
lium forms another kind of spore, a winter or resting spore, which 
carries the fungus through the winter and starts its life again 
in the spring. 
The White Rust. 
The white rust {Cystopus candidus) is very common on the 
shepherd’s purse. The mycelium grows in the leaves, stems and 
fruits, and forms cushions or masses of spores underneath the 
epidermis which by crowding burst the epidermis and set the 
spores free. With a lens examine these spots and note the rup¬ 
tured epidermis and the mass of white spores. These are sum¬ 
mer spores. Winter spores are formed which are similar to the 
winter spores of the downy mildews, and in fact these two kinds 
of fungi are closely related. 
