, 



PLATE 51. PICEA NIGRA. Tangential section through the spring wood showing 

 the structure of the two forms of medullary rays. X 46.8 



PLATE 52. PINTS MONTICOLA. (Type of soft pines, Sec. I.) Transverse section 

 showing the large and thin-walled tracheids of the spring wood, the very thin 

 and open summer wood, the large resin canals with the associated paren- 

 chyma broken out. X 46.8 



