104 THE STRUCTURE AND WORK OF THE STEM 



taken several years to grow, we find so-called annual rings. The 

 distance between one ring and the next (see Figure) usually rep- 

 resents the amount of growth in one year. Growth takes place 

 from an actively dividing layer of cells, known as the cambium 

 layer. This layer forms wood cells from its inner surface and 

 bark from its outer surface. Thus new wood is formed as a dis- 

 tinct ring around the old wood. 



Use of the Outer Bark. The outer bark of a tree is protective. 

 The cells are dead, the heavy woody skeletons serving to keep out 



Experiment to show that the skin of the potato (a stem) retards evaporation. 



cold and dryness, as well as prevent the evaporation of fluids from 

 within. Most trees are provided with a layer of corky cells. This 

 layer in the cork oak is thick enough to be of commercial impor- 

 tance. The function of the corky layer in preventing evaporation 

 is well seen in the case of the potato, which is a true stem, though 

 found underground. If two potatoes of equal weight are balanced 

 on the scales, the skin having been peeled from one, the peeled po- 

 tato will be found to lose weight rapidly. This is due to loss of 

 water, which is held in by the skin of the unpeeled potato. 



Passage of Fluids up and down the Stem. If any young grow- 

 ing shoots (young seedlings of corn or pea, or the older stems of 

 garden balsam, touch-me-not, or sunflower) are placed in red ink 



