118 DISPLAY OF LEAF-SURFACES [CH. 



For our purposes it must suffice to have established 

 the fact that leaves move, and that most of their move- 

 ments are connected with their continual striving to 

 use a word which, strictly speaking, conveys an erroneous 

 meaning, since we are dealing with unconscious life to 

 secure the best attainable position for exposure to light 

 and air. 



Some curious phenomena are intelligible when this 

 necessity for attaining the light-position is understood. 



It is a noticeable fact that the small leaves of our 

 ordinary trees, such as Beech. Hornbeam, Alder, Lime, 

 Poplars and Willows, and shrubs such as Dogwood, Spindle 

 Tree, Privet, Hippophae, &c, are not usually segmented ; 

 and that segmentation is practically confined to large 

 leaves such as Horse-chestnut, Ash, Walnut, Robinia, 

 Maples, &c. It would appear as if some relation between 

 the shading effect of leaf on leaf, and the size of the 

 lamina, may be traced here, though perhaps it is unwise 

 to generalise too far in this connection ; for there may 

 also be advantages secured in both by the display of many 

 small leaves on a shoot, and by the cutting up of the fewer 

 large ones into lobes or leaflets in connection with the 

 exposure of the surfaces to violent winds. 



Nevertheless it does appear significant that by having 

 numerous crowded narrow leaves with short internodes in 

 the spray e.g. Pines, Firs, Yew, Larch, Tamarisk, &c. 

 or fewer and larger, but still relatively many and small 

 simple leaves with somewhat longer internodes e.g. 

 Apple, Pear, Plum, Willows, &c. the same effects as 

 regards exposure to light and air are obtained as where 

 still fewer and larger compound leaves cut up into 

 numerous leaflets e.g. Robinia, Horse-chestnut, &c. 

 That is to say in all these cases we find the leaves or 

 leaflets, however arranged, are so displayed as to shade 

 each other as little as possible : the distances apart and 



