GALLS CAUSED BY FUNGI 117 



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. 



CoUosporium 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 

 pini. The spores are carried by wind-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 clavariaeforme (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 



