104 Sir John Lubbock [April 25, 



numerous in proportion than with us. Everybody must have noticed 

 how our deciduous trees are broken if we have snow early in the 

 season and when they are still in leaf. 



Evergreen leaves, such as those of the comparatively tough and 

 leathery oak and olive, are protected against animals by their 

 texture, and often, as in the holly, by spines ; they are better able to 

 resist the heat and dryness of the south than the comparatively tender 

 leaves of our deciduous trees, which would part too rapidly with 

 their moisture. It is perhaps an advantage to evergreen leaves to 

 be glossy, because it enables them better to throw off snow. More- 

 over, their stomata are often placed in pits, and protected with hair, 

 which prevents too rapid evaporation. The texture and structure of 

 leaves is indeed a wide and very interesting subject, but to-night I 

 must confine myself to the shaf)e. 



It is impossible to classify plants by the form of the leaf, which 

 often differs greatly in very nearly allied species. Thus, the common 

 plantain of our lawns (^Plantago major^ has broad leaves, P. lanceolnta 

 narrow ones. The width or narrowness of leaves depends on various 

 considerations. In herbaceous and stalkless plants, such as the 

 plantain, prostrate leaves tend to be broad, those which are upright 

 to be narrow. Thus, grasses, for instances, have more or less upright 

 narrow leaves. 



In other cases the width is determined by the distance between 

 the buds, and in others again by the number of leaves in a whorl. 



Cordate and Lohed Leaves. 



Among broad leaves we may observe two distinct types, according 

 as they are oval or palmate. Monocotyledonous j)lants, such as 

 grasses, sedges, lilies, hyacinths, very generally have upright and 

 narrow leaves. When they are wider, as, for instance, in the black 

 bryony, this is mainly at the base, where, consequently, the veins are 

 further apart, coming together again towards the apex. This we are 

 tempted, therefore, to regard as the primitive type of a broad leaf. 



There is, however, a totally different one, where the leaf is 

 palmate, like a hand, widening towards the free end. Here the veins 

 pursue a straight, diverging course ; and as they not only serve to 

 strengthen the leaf, but also to carry the nourishment, this is doubt- 

 less an advantage. Perhaps, however, the primary reason for this 

 arrangement is found in the fact that these leaves are generally folded 

 up, like a fan, while they are in the bud. 



I have elsewhere dwelt on the case of the beech, and perhaps I 

 may briefly refer to it again. The weight of leaves which a branch 

 can carry will of course depeud on its position and strength. The 

 mode of growth of the beech and the hornbeam are very similar, but 

 the twigs of the latter are slenderer, and the leaves smaller. If we 

 cut off a beech branch below the sixth leaf we shall find that the super- 

 ficial leaf area which it carries is about 18 square inches. But in our 



