Structure and Work of Stems 



37 



60. The Phloem Portion of the Stem is important, 

 because it is the channel through which the food sub- 

 stances are carried from the leaves to the roots. The 

 water moves up through the woody portion, but the 

 food material moves in 



the phlcem part of the 

 stem. When land is cleared 

 of large trees, the stumps 

 will continue to form water 

 sprouts for a long time, 

 unless the trees are first 

 "deadened." This is done 

 by cutting off the bark 

 entirely around the trunk 

 of the tree, thus leaving a 

 strip or girdle of the wood 

 exposed. This does not 

 cause the immediate death 

 of the tree, because water 

 can move up to the leaves 

 through the stems, 

 as before. How- 

 ever, no food can 

 pass down to the 

 roots, and they 

 finally die of star- 

 vation. When the 

 roots die, water is no longer absorbed, as the living 

 root -hairs are gone. Girdling kills trees by starving 

 the roots. (Fig. 26.) 



61. Roots May Die without Girdling. When fruit 

 trees overbear, nearly all the food formed in the leaves 

 goes to mature the fruit, and not enough goes down to 



WATER AND 

 SOLUBLE SUB5TANCE5* 



Fig. 26. Diagram to show the path of move- 

 ment of water and reserve food substances in 

 stemmed plants. 



