60 



ELEMENTS OF BOTANY. 



The green layer of the bark in young shoots does muclii 

 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. 



Fig 50 —Cross Section of Fir 



Wood. 



ii, a resin passage ; m, medullary 



rays. (Mucli magnified.) 



Fig. B1. — Longitudinal Eadial Section 

 through a Eapidly Growing Young 

 Branch of Pine. 

 t, t', t", bordered pits on wood-cells ; st, large 

 pits where medullary rays lie against wood- 

 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. 



