STEMS 105 



as it does to send it up through an iron pipe to the 

 same distance. As a matter of fact it takes more 

 force to carry it up in the tree because it must pass 

 through such very small vessels. While the whole 

 process may not be fully understood, yet some fea- 

 tures of it may be illustrated in the following 

 paragraphs. 



140. Boot or Needing pressure. — If the stump 

 of a tree which has been cut down in the spring is 

 examined it will be seen that the sap is oozing from 

 the cut surface in great volume, and if a cup is 

 placed under the cut end of a grapevine at this 

 season the amount of liquid thrown out may be 

 measured. This bleeding is due to a pressure exerted 

 by the living cells of the stem and roots, and it is 

 sufficient to force water to the tops of small plants 

 like the sunflower in the temperate zone and in trees 

 in the tropics, but in the United States it could not 

 send sap to the leaves of large trees. Then, again, 

 root pressure is strong only in early spring. 



141. The floiv of sap of the sugar maple. — Not 

 all flow of sap is due to bleeding pressure. Sugar 

 maples are tapped to obtain the sweet sap at a time 

 in the spring when the ground is still frozen. The 



