ON LEAVES. 



347 



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

 same form as the ordinary pear-leaf, they would be about seven inches 

 long by two to three in breadth. The mountain-ash, as we know, lives 

 in mountainous and exposed localities, and such a leaf would be unsuit- 

 able 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 in- 

 stance, varies greatly ; in the specimen tabulated, the stem was *13 in 

 diameter, and the area of the six upper leaves was sixty square inches. 

 In another, the six upper leaves had an area of rather over one hundred 

 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, etc., the distance from bud to bud varies com- 

 paratively little, and bears a tolerably close relation to the size of the 

 leaf. In the sycamore, maple, etc., 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 shall, I think, see the reason of their dif- 

 ferences of form. 



Let us begin, for instance, with the common lime (Fig. 1). The 

 leaf -stalks are arranged at an angle of about 40 with the branch, and 

 the upper surfaces of the leaves are in the same plane with it. The 



Fig. i. 



Fig. 2. 



result is, that they are admirably adapted to secure the maximum of 

 light and air. Let us take, for instance, the second or third leaf in 

 Fig. 1. They are four and a half inches long and very nearly as broad. 

 The distance between the two leaves on each side is also just four and 

 a half inches, so that they exactly fill up the interval. In Tilia par- 

 vifolia the arrangement is similar, but leaves and internodes are both 

 less, the leaves, say, one and a half inch, and the internodes *6. 



