io2 ROWAN. SYCAMORE. [CHAP. 



common Ash. But the ordinary leaves of a Pear are, 

 as we all know, simple and ovate, or obovate. Why, 

 then, should those of the Mountain Ash be so entirely 

 different? May, perhaps, some light be thrown on 

 this by the arrangement of the leaves ? They are 

 situated some distance apart, and though, as shown 

 in the table, they are small in comparison to the 

 diameter of the stem, still they attain a size of 15 

 square inches, or even more. Now, if they were of 

 the same form as the ordinary Pear leaf, they would 

 be about 7 inches long by 2 to 3 in breadth. The 

 Mountain Ash, as we know, lives in mountainous 

 and exposed localities, and such a leaf would be 

 unsuitable to withstand the force of the wind in such 

 situations. From this point of view, the division into 

 leaflets seems a manifest advantage. 



Perhaps it will be said that in some trees the leaves 

 are much more uniform in size than in others. This 

 is true. The Sycamore, for instance, varies greatly ; 

 in the specimen tabulated, the stem was "13 in diameter, 

 and the area of the six upper leaves was 60 square 

 inches. In another, the six upper leaves had an area 

 of rather over 100 inches, but in this case the diameter 

 of the stem was "18. 



Another point is the length of the internode. In 

 such trees as the Beech, Elm, Hornbeam, &c., the 

 distance from bud to bud varies comparatively little, 

 and bears a tolerably close relation to the size of the 

 leaf. In the Sycamore, Maple, &c., on the contrary, 

 the length varies greatly. 



Now, if, instead of looking merely at a single 

 leaf, we consider the whole bough of any tree, we 



