12 
BRITISH GALLS 
(notably R. hirsutum and R. ferrugineum) afford a striking 
example of this group. They are caused by Exobasidium 
Rhododendri. 
2. On a Sharply Defined Part of the Stem 
“ Galls arising from sharply defined parts of the stem 
are comparatively rare. One of the most remarkable is 
produced on the stems of a Laurel (Launts canaviensis) by 
the parasite Exobasidium Latin. When it appears above 
the bark it looks like an aerial root, but rapidly grows into a 
branched spongy body 8 to 12 cm. long, similar in appearance 
to one of the fungi belonging to the family Clavariae” 
(Kerner). The pronounced fusiform swellings on Juniper 
stems induced by Gymnosporangium clavariaefovme may also 
be included under this group (Plate XIX.). 
3. On the Roots 
To this section belong the tuberous masses on Alder roots 
caused by the hyphomycete Frankiella alni (Plate XVI.), 
and the well-known “finger and toe” disease of Cabbage 
roots caused by the myxomycete Plasmodiophova brassicae. 
4. Floral Organs Modified 
Galls belonging to this section are not common. 
Characteristic examples are (1) the remarkable green or 
reddish outgrowths—sometimes 30 mm. long—on the 
pistillate catkins of the Alder caused by Exoascus alnitorquus 
(figured in Chapter IX.), and (2) the curious hypertrophy 
of the ovaries in Primus domestic a, known as “ bladder 
plums,” caused by Exoascus pruni (Plate XXV.). 
5. Branches Modified 
To this group belong the curious malformations popularly 
known as “ witches’ brooms,” of which, perhaps, the best 
