388 Field, Forest, and Wayside Flowers 



the ends of its slenderest rootlets. Nature has taken 

 care that the water sucked in by the roots shall not 

 be evaporated and lost as it goes up through trunk 

 and branches to the thirsty little shoots at top. 

 So the year-old twigs have a tough skin, which is 

 nothing more nor less than a thin sheet of cork, 

 while older branches are encased by a thicker cork- 

 covering, which lies, as a rule, below the surface. 

 Be it thin or thick it is perfectly water-proof. 



The peel of a potato is nothing more nor less 

 than a layer of cork-cells, and, by observing the 

 quickness with which pared potatoes " dry out," 

 we realize how effective even a thin cork-covering 

 can be in preventing the transpiration of vegetable 

 moisture. 



The life-giving juices of the tree can not get 

 through the cork-layers of the bark to nourish the 

 outermost tissues of the trunk and branches. So 

 all these parts of the tree which lie outside the 

 cork-layer dry up, shrivel, crack apart, and at last 

 flake off and fall to the ground. 



These dried and drying tissues may include cells 

 of many sorts and sizes, which in their younger 

 days served various uses in the tree's domestic 

 economy. But now we speak of them all together 

 as the " outer bark.' 



