54 



PLANT BIOLOGY 



outside of the plant cylinder. It is evident, then, that the 

 covering of bark must expand in order to allow of the expan- 

 sion of the woody cylinder within it. The tis- 

 sues, therefore, must be under constant pressure 

 or tension. It has been determined that the 

 pressure within a growing trunk is often as 

 much as fifty pounds to the square inch. The 

 lower part of the limb in Fig. 61 shows that 

 the outer layers of bark (which are long since 

 dead, and serve only as protective tissue) have 

 reached the limit of their expanding capacity 

 and have begun to split. The pupil will now 

 be interested in the bark on the body of an old 

 elm tree (Fig. 62); and he should be able to 

 suggest one reason why stems remain cylindri- 

 cal, and why the old bark becomes marked 

 with furrows, scales, and plates. 



Most woody plants increase in diameter by the 

 addition of an annual layer or "ring" on the 

 BRANCH. outside of the woody cylinder, 

 underneath the bark. The monocotyledo- 

 nous plants comprise very few trees and 

 shrubs in temperate climates (the palms, 

 yuccas, and other tree- like plants are of 

 this class), and they do not increase 

 greatly in diameter and they rarely branch 

 to any extent. Consult the woodpile for 

 information as to the annual rings. 



Bark-bound Trees. If, for any rea- 

 son, the bark should become so dense 

 and strong that the trunk cannot ex- 

 pand, the tree is said to be "bark-bound." Such condition 

 is not rare in orchard trees that have been neglected. 



FIG. 62. PIECE OF 

 BARK FROM AN 

 OLD ELM TRUNK. 



