LEAF-ARRANGEMENT. 99 



sideration. Strength, for instance, is an important 

 element. Thus the Ailanthus, with a stem equal 

 in thickness to that of the Horsechestnut, 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 ex- 

 panse 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 puz- 

 zled 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 a 

 third 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 unrea- 

 sonable 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 ele- 



