EXCRESCENCES. 217 



Cork-wings are well known on the \oung 

 branches of Elms, Maples, etc., some varieties 

 of which have received specific names on this 

 account. 



Corky excrescences on leaves occur occasionall}' 

 in the Gooseberry, Holly and other plants, for 

 which no cause has been discovered. 



Lenticels are also formed on some leaf-galls, 

 and are remarkable as being structures not normal 

 on leaves. 



Pustules. This term may be emplov'ed gener- 

 ally for all slight upheavals of the surfaces of 

 herbaceous organs, which subsequently burst and 

 give egress to the spores, etc., of the organism 

 causing them, or merely fray away at the top if 

 no organism is discoverable. They are often due 

 to fungi e.g. Synchytj-iujii, Protomyces, Cystopus, 

 and Ustilagineae, and we may extend the use of 

 the general term also to those cases where the 

 stroma of the fungus itself bursts through the 

 cortex of older parts and forms the principal part 

 of the pustule e.g. Monilia, forming white or grey 

 pustules on Apples, Roestelia and other yEcidia. 

 forming yellow or orange pustules on leaves, etc. ; 

 Cucurbitaria and Nectria (red) breaking through 

 the cortex of trees, and Phoina and numerous 

 other Ascomycetes which form black cushions 

 Pustules on the leaves of Lysiinachia, Ajuga, etc. 

 are due to the parasitic Alga Phyllobium. 



Cylindrical stem swellings are caused by 

 Calyptospora : they are due to the hypertrophy 

 of the cortex of Bilberry stems permeated by 



