248 



THE MAINTENANCE OF THE INDIVIDUAL 



Wright Pierce 



The characteristic lenticels of the white birch 

 {Betiila populi folia). Note the placement of the 

 lenticles. 



Scattered over the surface of twigs and young tree trunks are 



found many lenticels, openings in the corky layer which become filled 



with loose masses of cells. 

 They are found both on 

 roots and stems and act 

 as pores which allow for 

 the exchange of gases be- 

 tween the living cells of 

 the cortex and the me- 

 dium outside. Lenticels 

 are often spoken of as 

 "breathing jDores" and 

 experimental evidence 

 seems to make this title 

 valid. 



As the stem or trunk 

 of a tree grows larger in 

 diameter, there is an in- 

 creasing area that uses 



water and foods. Cells cannot grow without food, and food in a 



growing plant cannot be made without water. The structures which 



put the water-conducting tissue of 



the inside of the stem in connection 



with the phloem of its outer part 



are known as vascular rays. They 



may be seen in almost any cross 



section of a tree which has 



produced secondary xylem and 



phloem. Here the cambium has 



rows of irregularly placed cells 



that instead of forming xylem and 



phloem produce ingrowing masses 



of more uniform parenchymatous 



cells making vertically placed 



strings of tissue. These bands 



act as conducting pathways 



for water from the xylem to 



the phloem and also as chan- 

 nels for elaborated food from the phloem to the xylem, thus dis- 

 tributing these materials to the growing trunk. Experiments by 



phi 



.oem 



(i:am\:)i.imri 





\ — •^yiein 



pith 



Note the bands of living parenchym- 

 atous tissue that grow inward toward 

 the pith. 



