THE LEAP AS A BUILDER. 13 



Next in order comes a compound leaf without 

 teeth, and following it one with teeth. (Figs. II and 

 I illustrate these two types perfectly.) Lastly comes 

 the horse-chestnut leaf, which has a radiating form 

 (see Fig. J), which is the extreme type of complexity 

 in a leaf. 



These types comprise all the leaves of trees out- 

 side of the pine family ; the needle leaves of the 

 latter are too simply formed to require explanations 

 beyond those given in the chapters devoted to the 

 evergreens. The possession of a simple method 

 whereby we may identify a tree by its leaf is a 

 stepping stone to a better knowledge of the tree 

 itself. It seems a strange fact that we do not fully 

 comprehend the great value of the billions and 

 billions of leaves that clothe the vast forests which, 

 as time progresses, are slowly disappearing before 

 the axe. The cubic feet of lumber which a tree 

 yields are not nearly as valuable to us as the leaves 

 which the living tree puts forth season after season. 



The greatest sphere of usefulness which a tree 

 occupies is connected with its life. It is a great air- 

 purifier ; it absorbs from the atmosphere the carbonic- 

 acid gas which is poisonous to us ; it holds and slowly 

 dispenses moisture which the parched air needs ; it 

 gives out the ozone (or oxygen in an active electro- 

 negative condition) which is peculiarly conducive to 



