The Structures and Processes of Stems 135 



are being lifted in trees during the summer. If at this season 

 a hole is bored into the tnmk of a tree and an air-tight con- 

 nection made between this hole and a tube that has its lower 

 end in a vessel of water, the water is drawn into the stem, not 

 forced out. This indicates that there is more pull on the water 

 from above than there is pressure from below. It is known 

 also that there may be currents moving downward in one 

 layer of the wood and upward in another, although the gen- 

 eral direction of water transport is upward to the leaves. It 

 is certain that the roots do not force the water up into the 

 tops of trees. 



The primary factor, then, in the rise of sap is transpiration ; 

 the second factor is the drawing of water from the water- 

 conducting tissue by the mesophyll ceUs to replace that lost 

 through transpiration; the third factor is the cohesion of 

 water columns in the long strands of water-conducting tissue, 

 which makes it transmit the pull from the mesophyll cells 

 all the way down to the roots. In the chapter on roots we 

 shall learn how the water passes from the soil into the roots, 

 and to what extent the roots aid in the lifting of water. 



The pulling up of water by transpiration is exemplified 

 when cut flowers are placed in a vase containing water. That 

 water is drawn up into the flowers may be shown by placing 

 the stems of white flowers in water colored with red ink. 



The flow of maple sap. The water in stems always con- 

 tains a certain amount of sugar in addition to mineral salts. 

 In the maple, in the early spring when the days are warm but 

 freezing still occurs at night, great quantities of sugar pass 

 into the water-conducting tissue. This sugar comes from the 

 pith rays and other tissues where it accumulated in the form 

 of starch during the preceding growing season. With the 



