35 



THE POPULAR SCIENCE MONTHLY. 



angles to the axis of the branch instead of being parallel to it, have 

 long petioles, and palmate instead of pinnate veins. In this group the 

 mode of growth is somewhat stiff ; the main shoots are perpendicular, 

 and the lateral ones nearly at right angles to them. The buds, also, 

 are comparatively few, and the internodes, consequently, at greater 

 distances apart, sometimes as much as a foot, though the two or three 

 at the end of a branch are often quite short. The general habit is 

 shown in Figs. 9 and 10. Now, if we were to imagine six beech or 



Fig. 9. 



Fig. 10. 



elm leaves on these three internodes, it is obvious that the leaf surface 

 would be far smaller than it is at present. Again, if we compare the 

 thickness of an average sycamore-stem below the sixth leaf with that 

 of a beech-stem, it is obvious that there would be a considerable waste 

 of power. Once more, if the leaves were parallel to the branch, 

 they would, as the branches are arranged, be less well disposed with 



reference to light and air. A glance at 

 Figs. 9, 10, and 11, however, will show 

 how beautifully the leaves are adapted 

 to their changed conditions. The blades 

 of the leaves of the upper pair form an 

 angle with the leaf-stalks, so as to assume 

 a horizontal position, or nearly so ; the 

 leaf-stalks of the second pair decussate 

 with those of the first, and are just so 

 much longer as to bring up that pair 

 nearly, or quite, to a level with the first ; 

 the third pair decussate with the second, 

 and are again brought up nearly to the same level, and immediately to 

 the outside of the first pair. In well-grown shoots there is often a 

 fourth pair on the outside of the second. If we look at such a cluster 

 of leaves directly from in front, we shall see that they generally appear 



Fig. 11. 



