34 6 



THE POPULAR SCIENCE MONTHLY. 



In the elm the numbers are *11 and 34, in the chestnut "15 and 72, 

 and in the horse-chestnut the stem has a thickness of *32, and the six 

 leaves have an area often of three hundred square inches. Of course, 

 however, these numbers are only approximate. Many things have to 

 be taken into consideration. Strength, for instance is an important 

 element. Thus the ailantus, with a stem equal in thickness to that of 

 the horse-chestnut, carries a smaller area of leaves, perhaps because it is 

 less compact. Again, the weight of the leaves is doubtless a factor in 

 the case. Thus in some sprays of ash and elder which I examined of 

 equal diameter, the former bore the larger expanse of leaves ; but not 

 only 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 one third as long. May this not perhaps be due 

 to the fact that they remain on 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 posi- 

 tion of the leaves, the direction of the bough, and many other elements 

 would have also to be taken into consideration, 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 on the form of the leaf also. 



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 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 not, perhaps, some 

 light be thrown on this by the arrangement of the leaves ? They are 



