110 THE NATURE AND WORK OF PLANTS 



obtained. Perhaps the best method is to drive two 

 nails into the opposite sides of the trunk of a vigor- 

 ous young poplar in early spring, and then find 

 the exact circumference of the tree an inch above 

 these nails with a tape measure. Repeat the opera- 

 tion of measurement about the first of September. 



150. The hark. — The behavior of bark should 

 be studied in connection with growth in thick- 

 ness. The yoimg poplar and many other young 

 trees are seen to have a smooth bark, while older 

 specimens have a very rough or even a shaggy 

 covering. The cells which compose the bark of the 

 younger trees are alive, and divide and grow in such 

 manner that they keep pace with the increase of 

 the trunk. After the tree reaches a certain age 

 the bark does this no longer. Any increase in the 

 trunk then results in the splitting of the bark, 

 leaving the edges exposed. A new layer of bark 

 is formed which is applied to the inside of this slit 

 like a patch. This process is repeated every year, 

 an,d as a consequence some trees, such as the oaks, 

 hickories, and poplars, have a very rough or shaggy 

 bark. The sycamore is an example of a tree which 

 casts away the old layer of bark each year and 



