100 ANATOMY OF STEMS, ROOTS, AND LEAVES 



cbjma cells aud are a coutiiiuation of the pith cells ; but in older 

 stems they are frequently compressed into extremely thin, flat 

 layers of cells separating the fibro-vascular bundles. 



The bundles increase in mnnber with the age of the plant. 

 In young stems the bark is soft and greenish in color and can 

 be readily separated from the underlying woody parts. In the 

 older stems the outer bark is dead and frequently cracked and 

 conceals the soft or true bark. In peeling the stem the separa- 

 tion sometimes occurs in the cambium. The increase in the 

 number of cells in the dicotyledonous stems is confined almost 

 entirely to the cambium layer of cells which produces new cells 

 for both the xylem and the phloem. Therefore, it will be readily 

 seen that the formation of the new layers of xylem cells results 

 in increasing the diameter of the stem without increasing the 

 inner diameter of the cylinder of bark. This increase of the size 

 of the stem must necessarily exert a considerable strain on the 

 bark covering, which results in the very slow splitting and peel- 

 ing of the outer bark. This explains the roughness of the bark 

 of most of our trees and the characteristic natural peeling of 

 many others. As the old, outer bark is gradually shed, the 

 newly formed inner bark becomes the outer and new layers 

 are formed from below. 



Cause of Annular Rings. — Tke formation of the cells of the 

 xylem is not uniform throughout the season. In our northern 

 climate, the cells formed in the first part of the growing season 

 are larger and have thinner walls than those formed later. This 

 difference causes the so-called annular rings. 



Sap-Wood.- — The light-colored sap-wood is composed of cells 

 in which there is little or no growth, but which contain more 

 or less protoplasm. 



Heart-Wood. — The dark-colored heart-wood is composed of 

 cells from which the protoplasm has disappeared. They are 



