v.] STRENGTH OF STEM. 101 



the Horse Chestnut, carries a smaller area of leaves, 

 perhaps because it is less compact. Again, the 

 weight of the leaves must doubtless be taken into 

 consideration. Thus in some sprays of Ash and 

 Elder of equal diameter, which I examined, the 

 former bore the larger expause of leaves ; not only, 

 however, is the stem of the Elder less compact, but 

 the Elder leaves, though not so large, -were quite as 

 heavy, if not indeed a little heavier. I was for some 

 time puzzled by the fact that, while the terminal 

 shoot of the Spruce is somewhat thicker than that of 

 the Scotch Fir, the leaves are not much more than $ 

 as long. May this not perhaps be due to the fact that 

 they remain on the tree more than twice as long, so 

 that the total leaf area borne by the branch is greater, 

 though the individual leaves are shorter ? Again, it 

 will be observed that the leaf area of the Mountain 

 Ash is small compared to the stem, and it may, 

 perhaps, not be unreasonable to suggest that this 

 may be connected with the habit of the tree to grow 

 in bleak and exposed situations. The position of the 

 leaves, the direction of the bough, and many other 

 elements would have also to be taken into consi- 

 deration, but still it seems clear that there is a 

 correspondence between thickness of stem and size 

 of leaf. This ratio, moreover, when taken in relation 

 with the other conditions of the problem, has, as we 

 shall see, a considerable bearing not only on the size, 

 but also on the form of the leaf. 



The Mountain Ash has been a great puzzle to me ; 

 it is, of course, a true Pyrus, and is merely called Ash 

 from the resemblance of its leaves to those of the 



