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 {Laurus canariensis) by 

 the parasite Exobasidium Lauri. 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 clavariaeforme 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 FmnMella alni (Plate XVI.), 

 and the well-known "finger and toe" disease of Cabbage 

 roots caused by the myxomycete Plasmodiophora brassicae. 



4. Floral Organs Modified 



Galls belonging to this section are not common. 

 Characteristic examples are (i) 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 Prunus domestica, 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 



