104 



SOILS AND PLANT LIFE 



starves the roots. A girdled tree does not sprout from the 

 stump. 



80. The Flow of Sap. — We have perhaps heard that 

 the sap flows upward in the spring and back downward 

 in the fall. This is not the case. The roots absorb water 

 throughout the growing season and this water moves up- 



FiG. 33. — How a graft is made and how it grows together. 



ward rather rapidly through the water-carrjdng tubes 

 to the leaves where food is manufactured ; and throughout 

 the growing season, the food manufactured in the leaves 

 oozes back downward through the sieve tubes to all parts 

 of the plant. 



81. The Cambium Layer. — In the dicotyledons, there 

 is a layer of cells between the water-carrjdng vessels and 

 the sieve tubes which lie outside of them. This is known 

 as the cambium layer. It is the thin, wet, slippery layer 



