VIL] THE LEAVES. 63 



Elm. In the Oak, Ash, and Pine, both terminal and 

 axillary buds annually develop branches. 



This variety of conditions in respect to the relations of 

 terminal and axillary buds necessarily influences the 

 general aspect, or habit, of the tree.- Note, for ex- 

 ample, the contrast, in habit, between the Beech and 

 the Elm. The leaves are two-rowed (distichous), the 

 nodes often equally numerous, and the internodes about 

 equidistant in length in both. But while in the Beech 

 the axillary branchlets from the same branch of the 

 previous year are nearly equal in length, in the Elm they 

 tend to become successively longer towards the apex, so 

 that the contour of a two-year-old twig of Beech is com- 

 paratively parallel-sided and narrowed above, of Elrn 

 much broader, or obovate, above. The persistence of 

 this general tendency year after year results in the con- 

 trast so conspicuous in winter between the two trees ; 

 between the arrow-like extremities of Beech, the rounded 

 or shield-like terminal masses of Elm. In this connec- 

 tion, having regard also to the alternation and mutual 

 relation of leaf-bearing and flower-bearing nodes, our own 

 British forest trees offer much interesting material for 

 study, the normal relations being generally most clearly 

 discernible in young and healthy examples. 



Leaves vary in their duration. In our climate they 

 usually last but one season ; at the close they separate 

 from the stem at a definite transverse plane, leaving a 

 scar ; if they remain attached, they decay gradually. In 

 Evergreens, the leaves either persist until the expansion 

 of those of the second year, then falling off, in which case 

 they are merely of annual duration, or they may remain 

 attached two, three, or more years, in which case the same 

 spray exhibits the successive generations, which are 

 usually separated from each other by a space of twig 

 bearing only reduced or scale-like leaves, answering to 

 the winter-breaks between the successive periods of full 

 leaf-development. 



6. In the fully developed leaf we have already distin- 

 guished petiole and blade. The mode in which the blade 

 is folded while enclosed in the bud is spoken of as the 

 vernation of the leaf. 



The blade is divided longitudinally into symmetrical 



