THE STALK OF THE PLANT 71 



on one side a layer of wood which attaches itself to 

 the outer surface of the preceding year's layer, and 

 on the other side a thin sheet of bark which is added 

 to the inner surface of the bark already formed. 



"Thus each year both bark 

 and wood form a new layer ; but 

 this added layer is applied in 

 opposite ways in the two in- 

 stances, — outside on the wood, 

 inside on the bark. The wood 

 thus encircled from year to year 

 by new layers increases in age o«»s section of Tree Trunk 

 toward the center and becomes younger and younger 

 toward the circumference, whereas the bark, lined 

 every year with a fresh sheet, shows its youth on the 

 inside and its age on the outside. The first buries 

 inside the trunk its decrepit and dead layers; the 

 second thrusts its old layers outside, where they 

 crack and fall off in large scales. This aging process 

 is simultaneous on the outside and in the center of 

 the tree-trunk; but between the wood and the bark 

 life is always at work, creating fresh accretions. 



"Here are some experimental proofs of this an- 

 nual formation of a ligneous layer. A strip of bark 

 is removed from the trunk of a tree, and on the 

 wood thus laid bare is fastened a thin sheet of metal. 

 The bark is then replaced and bound with ligatures 

 so that the wound may heal. We will suppose ten 

 years have passed. The bark is raised again at the 

 same place. The metal sheet is no longer visible ; 

 to find it you must bore deep into the wood. Now, 



