1921] WHITAKER—BIRCH AND OAK 235 
Fic. 16.—Transverse section of wounded seedling stem of B. papyrifera. 
Fic. 17.—Part of same in wound cap, more highly magnified, showing 
diffuse type of ray structure. 
Fic. 18.—Part of same opposite wound, showing aggregate character of 
rays. 
PLATE XV 
Fic. 19.—Transverse section of root of Quercus rubra. 
Fic. 20.—Part of same more highly magnified, illustrating aggregate 
character of ray organization and showing transformation into parenchyma 
taking place in central part of ray 
Fic. 21.—Tangential section through root of Q. rubra, showing aggregate 
ray structure. 
Fic. 22.—Transverse section of wounded stem of Q. velutina. 
Fic. 23.—Part of same through wound cap, more highly magnified to 
show cage rays. 
Fic. 24.—Transverse section through wound cap of another specimen of 
same species, showing transformation from aggregate to compound type of 
ray organization. 
