70 



LESSONS WITH I'LANTS 



wood) under the bark, and we know that in all 

 plants the growth in thickness takes place upon 

 the inside of the cylinder, and not upon 

 the very outside. 



73. It is evident, then, that the cover- 

 ing of bark must expand in order to 

 allow of the expansion of the woody 

 cylinder within it. The tissues must, 

 therefore, be under 

 constant pressure or 

 tension. It has been 

 determined that the 

 pressure within a 

 growing trunk is oft- 

 en as much as fifty 

 pounds to the square 

 inch. 



74. A piece of an 

 elm branch ten years 

 old is drawn in Fig. 

 ^'«- "»■ 68. It is an inch in 



^''"^ "IJr "" "" diameter, yet the 

 bark at the top is 

 smooth and intact. At one time, the shoot was 

 not more than one-eighth of an inch in diameter at 

 this point. The pupil may now figure out how 

 much this bark has expanded by the combined 

 action of intercalary growth and stretching. 



Fi8. 68. 



Cracking of the 

 bark on an elm 

 branch. 



