CHAPTER III 



THE HEALING OF WOUNDS 



Let us consider for a moment the general 

 make-up of the plant cylinder. The 

 young shoot is tightly enveloped with 

 bark. We observe that in many 

 plants the increase in diameter of the 

 stem comes about by the formation 

 of rings of new tissue (or new wood) 

 under the bark, and we know that in 

 all plants the growth in the thick- 

 ness takes place upon the inside of 

 the cylinder, and not upon the very 

 outside. It is evident, then, that 

 the covering of bark must expand in 

 order to allow of the expansion of 

 the woody cylinder within it. The 

 tissues must, therefore, be under con- 

 stant 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. 



A piece of an elm branch ten years 

 old is drawn in Fig. 68. It is an 



68. Cracking of 



the bark on an 



elm branch. 



(76) 



