36 



Elementary Principles of Agriculture 



bundles, but with age become so numerous that they 

 unite to form the solid woody portion of the stem. 

 Outside of this woody region is a layer of very thin- 

 walled cells that are actively dividing and growing. 

 This is the cambium layer. (Fig. 24c ) 



58. Cambium. The cambium is the region of active 

 growth in the stem of plants with netted veined leaves. 

 It causes the stem to increase in diameter by adding 

 layers of cells each season, forming the annular rings. 



(Fig. 25.) The 

 cambium cells 6n 

 the inner side be- 

 J come wood cells 

 and water ducts, 

 while the cells on 

 J the outside are 

 gradually trans- 

 formed into the 

 food - conduct- 

 i n g channels, o r 

 phloem, just under the bark. The increasing thicken- 

 ing of the stem breaks the outer bark in long, vertical 

 slits, and new bark is formed below. 



59. Wounds made by pruning, gnawing of rabbits, 

 breaking of branches, and other agencies, are often 

 healed over by the growth of the cells of the cambium. 

 Whenever the cambium cells form an extra growth in 

 this way, it is called callus. Where large limbs are 

 removed, it takes several years for the callus to grow 

 over the wound. When trees are pruned, the exposed 

 part should be heavily painted, to protect it till the 

 callus can have time to grow over entirely. (See H 186, 

 How to Make the Cuts in Pruning.) 



Fig. 25. Cross-section of an oak stem, showing 

 the "annular" rings at J, which mark the 

 close of the growing season. 



