2i8 DISEASE IN PLANTS. 



the hyphae. Epicliloe, which clothes the sheaths 

 and halms of grasses with its stroma, at first 

 snowy white and later ochre-yellow as the peri- 

 thecia form, is another example. 



The cylindrical layer of eggs of a moth such 

 as Bonibyx on a twig must not be confounded with 

 these cases. 



Frost-blisters are pustule-like uprisings of the 

 cortex, where the living tissues below have formed 

 a callus-like cushion into the cavity beneath the 

 dead outer parts of the cortex which were killed 

 by the frost ; they occur on the stems of young 

 Apples, Pears, etc. 



Galls in the narrower sense are tissue out- 

 growths usually involving deeper cell-layers. They 

 are so varied and numerous that classification is 

 difficult. For symptomatic purposes we may 

 divide them as follows : 



Leaf-galls. A well-marked type is that of the 

 pocket-galls or bladders in which the whole thick- 

 ness of the leaf is as it were pushed up like a 

 glove-finger at one spot, so that if the upper 

 surface of the leaf forms the outside of the gall 

 the lower surface is its lining. Such galls are 

 common on Limes {Phytoptus), Glechoma {Ceci- 

 domyia), Elms {Tetraneiira), etc. Similar localised 

 extension of the leaf surface, compelling it to rise 

 up like a pocket, are caused by fungi e.g. Taphrina 

 on Poplars, Exoascus on Birches, etc., Exobasidiuni 

 on Bilberries, Rhododendrons, etc. 



Another type is that of the Gall-apple^ so well 

 known on Oaks, where the spherical swelling is 



