58 



Introduction to Botany. 



tracheal tubes are left intact, since the bark separates at 

 the cambium ring (see Fig. 21). This accounts for the fact 

 that the leaves do not wither when the bark is removed by- 

 girdling. 



While water is being supplied through the roots the 

 most important process of, the manufacture of food mate- 



Diagrammatic representation of the structure of a Stem, 

 the bark being partly stripped off at the cambium ring. 

 and p, tracheal tubes, which carry the water upward 

 from the roots ; thin places are seen in the walls of these tubes in the form of 

 pits and rings, w and x, sieve tubes which carry food down from the leaves, or 

 up, as needed, v, medullary ray, which carries water and food radially to and 

 from the bark and the "wood as needed, s, bast fibers. Cells of the cambium 

 ring are clearly shown to the right of the sieve tube, .t. 



rials is taking place in the leaves, and highways must be 

 provided for the transport of these food materials down- 

 ward and upward wherever growth in length or in thick- 

 ness is taking place. The tracheal tubes could not answer 

 this purpose because the upward movement of the water 

 through them would prevent the passage of food materials 



