GALLS CAUSED BY FUNGI 
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leaf by a narrow band, never exceeding 2 mm. in width, and 
the leaf is seldom distorted. These galls also occur on the 
stems and petioles. 
Coleosporium senecionis is a typical example of a heteroecious 
fungus. Its aecidiospores are produced on the needles and 
young branches of the Scots Pine in May and June; the 
perennial mycelium causes pronounced fusiform swellings 
on the branch. In this stage it is known as Peridermium 
pint. The spores are carried by w T ind-currents to the 
Groundsel and to various Ragworts ( Senecio viscosus , sylvaticus, 
and Jacobaea ), and, germinating thereon, enter the tissues, 
and give rise throughout the year to uredospores and 
teleutospores, which appear in orange-coloured pustules on 
the leaves. 
Another interesting heteroecious fungus gall-causer infests 
the Common Juniper. Juniper bushes in April and May not 
infrequently have large fusiform swellings on the branches, 
from which project compressed, elongated, pale orange 
bodies, 10 to 12 cm. long, which are firm and horny at first, 
becoming gelatinous. These bodies contain the dark yellow 
teleutospores of Gymnosporangium clavaviaeforme (Plate XIX.). 
These spores are carried to Hawthorn bushes, and, alighting 
on a damp leaf or other growing part, germinate and enter 
the tissues. In about a month’s time the spores (aecidiospores) 
of the second form of the fungus appear. These are carried 
back by wind, insects, or other agency, to the Juniper bushes, 
where they germinate, obtain entrance, and ultimately give 
rise to the teleutospores. The mycelium is perennial in 
the branches. 
In the “witches’ brooms,” the dense masses of twigs 
seen on the branches of many trees, there is a suggestion 
of symbiosis. The parasite stimulates growth at the point 
of infection. The branches of the “broom” are more or 
less erect; its leaves are usually reduced in size, often lack 
chlorophyll, and the fruit-bodies (“cluster cups”) of the 
fungus appear on them. We have already observed that the 
“ witches’ brooms ” so frequently seen on the Birch often 
