136 Science of Plant Life 



coming of warmer weather the starch is digested, and the sugar 

 formed from it diffuses into the water-conducting vessels. 



The earlier sap is the richest and apparently comes largely 

 from the upper parts of the tree. The last sap is more dilute 

 and probably comes from the roots. The positive pressure 

 that produces the flow occurs only during the day ; at night 

 it becomes negative and the sap flow ceases. The causes of the 

 pressure are only partly known. A portion of it is due to 

 the expansion of gas bubbles within the tree, but this gas 

 expansion accounts for only a small part of the pressure. 



Whether the flow shall continue for weeks or stop after a 

 few days is determined by weather conditions ; but just how 

 the ' several weather factors like changes in temperature or 

 rainfall bring about the increase and decrease of pressure, is 

 unknown. Even under the most favorable conditions it is 

 not possible to draw out of a tree more than 5 per cent of the 

 food that it contains. 



A flow of sap somewhat similar to that in the maples occurs 

 in many species of trees, as in the birch, butternut, and 

 hornbeam. Sugar and sirup are made from the hard maples. 



PROBLEMS 



1. A tree increases in height at the rate of 2 feet a year. When the tree is 

 5 years old, a nail is driven into the trunk 2 feet from the ground. How far 

 from the ground will the nail be when the tree is 10 years old? 



2. Why is the timber from monocot stems less useful for building purposes 

 than that from dicot stems? 



3. What are the woods principally used in your locality for interior finishing 

 and for making shingles, posts, and flooring? What properties do these 

 woods have that fit them for these special uses? 



4. What kinds of wood are used in the making of furniture ? Why? 



5. How is a log quarter-sawed? Why is quarter-sawed lumber more valuable 

 than that cut otherwise? 



