60 ELEMENTS OF BOTANY. 
The green layer of the bark in young shoots does much 
toward collecting and preparing the food of the plant from. 
air and water, but this work may be best explained in connec- 
tion with the study of the leaf, Chapter XII. 
Oy y 
ia 
as ( 
oe: 
Fic. 51. — Longitudinal Radial Section 
m mM through a Rapidly Growing Young 
Fie. 50.— Cross-Section of Fir Branch of Pine. 
Wood. t, t', t’, bordered pits on wood-cells; st, large 
8S, aresin passage ; m, medullary pits where medullary rays lie against wood- 
rays. (Much magnified.) cells. (Much magnified.) 
Finally, the corky layer of the bark serves to a considerable 
extent as a protection against sudden changes of temperature 
and aids greatly in preventing evaporation of water on ‘its 
way along the stem. 
re geter- 
