PLANT PHYSIOLOGY AS RELATED TO PRUNING 



21 



and, as the tissues become old, they gradually cease to 

 function in water and food conduction. Thus in trees 

 and shrubs the interior tissues serve only to strengthen 

 the stems. In other words, the "heart wood" is dead. 



The earliest formed of the vascular bundles are ar- 

 ranged in a more or less circular series ; those formed 

 later are developed from the cambium between them. 

 Ultimately such large numbers of these bundles may be 

 formed that in woody and semi-woody dicotyledonous plants 

 they may form a cylinder of wood, though the indi- 

 vidual bundles may be partially or wholly separated from 

 each other by thin plates of wood called medullary rays 



FIG. 15 FIBRO-VASCULAR BUNDLE 



A, cross section, phloem showing sieve tubes, a, companion cell, e, scleren- 

 chyma and g, parenchyma, b, xylem, c, showing vessels, d, between the two cam- 

 bium, /; B, longitudinal section. Outside cells parenchyma. Letters in A apply to 

 various tissues. 



(Fig. 14). Since the cambium in such plants produces 

 new xylem on its interior, it is steadily moved annually 

 or seasonally farther outward from the center of the 

 stem ; since new phloem is developed on the outer side 

 of the cambium, both the wood and the bark increase in 

 thickness and the stems, as a whole, increase in girth ; 

 and since the xylem developed in the early part of the 

 season is thinner walled than that produced later when 

 growth is not so active, the characteristic annual or 

 annular rings so readily seen in cross sections of dicoty- 

 ledonous woody plants are formed. 



