TRANSFER OF MATERIALS IN PLANTS 



177 



dead. Although water and salts may still be able to reach the 

 upper parts of the plant (since the channels that served during 

 the previous season are still open), the food that should have 

 been accumulated during the previous summer is lacking. 



A 



FIG. 65. Bark fibers and vessel 



A, a section cut lengthwise, and , 

 one cut crosswise, showing bast fibers, 

 sieve-plate vessel, and the so-called 

 companion cells found next to the 

 sieve-plate cells 



207. Circulation of sap. In 



plants there is no circulation 

 of materials such as we find in 



FIG. 66. Ascent and descent of sap 



The arrows in the diagram are to show 

 that materials absorbed by the roots travel 

 upward through vessels located in the 

 wood part of the stem, and that ma- 

 terials resulting from the food-making 

 processes in the leaves travel downward 

 through vessels located in the bark. 

 When a complete ring of bark is removed 

 from a tree, the plant may live on to the 



the higher Classes Of animals. end of the summer ; but the buds will not 

 . _ . open the following spring, since they de- 



There is, it is true, a move- pend upon food accumulated in the roots 



ment of liquid from the root 



to the leaves, and from the leaves to the roots ; but the matter 

 conducted along the two sets of vessels is not the same either 

 in amount or in kind. 



The water that moves from the roots to the leaves is 

 several times as great in quantity as the liquid that moves 



