THE STRUCTURE AND WORK OF THE STEM 105 



Apple twigs split to show the 

 course of colored water up 

 the stem. 



(eosin), left in the sun for a few hours, and then examined, the red 

 ink will be found to have passed up the stem. If such stems were 

 examined carefully, it would be seen that 

 the colored fluid is confined to the collec- 

 tions of woody tubes immediately under 

 the inner bark. Water evidently rises in 

 that part of the stem we call the wood. 



But if willow twigs are placed in water 

 roots soon begin to develop from that 

 part of the stem which is under water. 

 If now the stem is girdled by removing 

 the bark in a ring just above where the 

 roots are growing, the latter will even- 

 tually die, and new roots will appear 

 above the girdled area. The food ma- 

 terial necessary for the outgrowth of 

 roots evidently comes from above, and 

 the passage of food materials takes place 

 in a downward direction just outside 

 the wood in the layer of bark which contains the bast fibers and 

 sieve tubes. Food substances are also conducted to a much less 

 extent in the wood itself, and food passes from the inner bark to 

 the center of the tree by way of the pith plates or medullary 

 rays. This can be proved by testing for starch in the medullary 

 rays of young stems. It is found that much starch is stored in 

 this part of the tree trunk. This experiment with the willow 

 explains why it is that trees die when girdled so as to cut the 

 sieve tubes of the inner bark. The food supply is cut off from 

 the protoplasm of the cells in the part of the tree below the cut 

 area. Many of the canoe birches of our Adirondack forest are 

 thus killed, girdled by thoughtless visitors. 



In What Form does Food pass through the Stem? We have 

 already seen that materials in solution (those substances which will 

 dissolve in the water) will pass from cell to cell by the process of 

 osmosis. This is shown in the experiment illustrated on the 

 following page. Two thistle tubes were partly filled, one with 

 starch and water, the other with sugar and water, and a piece 

 of parchment paper was tied over the end of each. The lower 



