20 



AMERICAN MUSEUM GUIDE LEAFLETS 



[irregularity of the wood cylinders in "burls," overgrown knots and excres- 

 cences occasional!} Found on various trees, causes the unusual and often 

 very beautiful effects seen in burl veneers I Fig. 15). 



It i> the work of these various tissues in cooperation for the tree's 

 maintenance as an individual that is of most interest. Water containing 

 lime, potash and other minerals in solution, i- taken up from the soil 

 through the root-. This food cannot In- used directly for growth by the 

 tree any more than it could he by man. It is -cut upward through the 

 sapwood to the leaves of the tree. A tree may he girdled, that i>. cut to 

 the cambium, and also may he hollow through the whole extent of its 

 heartwood, yet it will live for some time if there is continuous sapwood 



Light 



Upper surface 

 of leaf 



Wattr 



y Inner bark 

 / (bast) 



Wood 



FIG. 17. ACTIVITIES GOING ON IN THE "CELLS" AND AIR SPACES OF A LEAF 



1 . Under the sun's heat, water i- continually evaporating from the leaf. A 

 tree must lift several thousands of lbs. of water to the leaves to gel 1 lb. of mineral 

 matter in solution in the sap. The greater part of this water i> evaporated from the 

 leaves A birch tree gives off 700 1000 lbs. of water daily; a single oak sends off 

 into the air 130 tons of water annually 



2 , Carbon dioxide is absorbed continually in daylight by the leaf to make 

 possible the starch-forming process, while free oxygen i- at the same time given off 

 .-,. ,-, waste producl from this same process. This absorption of carbon dioxide 

 and outpouring of oxygen makes one explanation of the fact thai park and street 

 in-.- increase the healthfulness of a citj 



