﻿Bui. 1131, U. S. Dept. of Agriculture. 



Plate IV. 



Frost Rings of Pinus densiflora, Abies grandis, and Tsuga hetero- 



PHYLLA. 



A . — Transverse section of stem of Pinus densiflora taken 10 centimeters below the 1921 whorl of 

 branches, showing a slight frost ring formation in the 1921 growth. (X135.) B. — Transverse 

 section of Abies grandis, showing a more pronounced frost-ring formation, with the character- 

 istic lateral displacement of the medullary rays and their proliferation. (X135.) C. — Trans- 

 verse section through a frost-ring formation in Tsuga heterophylla, showing the formation of 

 a broad zone of parenchyma wood (X135.) 



