CH. IV, 2] STRUCTURE OF STEMS 123 



a few days, by formation of cork layers, often manifest by 

 their brown color. Where an injury includes the wood, as in 

 case of broken branches or the pruning of large trees, the 

 wood itself does not heal, but the neighboring bark, and also 

 the cambium, gradually overgrows it. In time the cambium 

 reestablishes itself over the injury and then continues to make 

 wood as before (Fig. 79). This power of healing injuries 

 has high value for plants, since their epidermis and cork 

 form not only a protection against dryness, but serve also 



: 



_ 



::; nF!L^, 



FIG. 80. Cross section through bark and wood of an old Elm tree, 

 showing abscission of the bark ; X f . 



as their first line of defense against the entrance of injurious 

 parasites, which are ever ready to enter any break in the 

 tissues of the stem. 



With increasing age several new features appear in woody 

 stems. Sections then show that the outer part of the bark, 

 which is dead, is cut off from the interior living part by 

 layers of cork, which form anew each year, much as the 

 absciss layers form in the bases of leaves (Fig. 80). As in case 

 of leaves, also, the valuable materials in the outer bark are 

 previously removed to the stem. This dead bark becomes 

 vertically cracked by the pressure of the expanding wood 

 within, and the resultant fissures replace the lost lenticels 

 as avenues of gas exchange between the interior of the stem 



