198 Sir John Luhhock [Feb. 13, 



Arrautjement. 



Moreover, not only do the leaves on a single twig admirably fit 

 one another, but they are also adapted to the ramification of the twigs 

 themselves, and thus avail themselves of the light and air, as we can 

 see by the shade they cast without large interspaces or much over- 

 lapping. In the Sycamores, Maples, and Horse Chestnuts, the 

 arrangement is altogether different. The shoots are stiff and upright, 

 with leaves placed at right angles to the plane of the branch, instead 

 of being parallel to it. The leaves are in pairs, and decussate with 

 one another, while the lower ones have long petioles, which bring 

 them almost to the level of the upper pairs, the whole thus forming a 

 beautiful dome. 



For leaves arranged as in the Beech, the gentle swell at the base 

 is admirably suited, but in a crown of leaves, such as those of the 

 Sycamore, space would be thereby wasted, and it is better that they 

 should expand at once, as soon as their stalks have carried them free 

 from the upper and inner leaves ; hence we see how beautifully the 

 whole form of these leaves is adapted to the mode of growth and 

 arrangement of the buds in the jDlants themselves. 



In the Black Poplar the arrangement of the leaves is again quite 

 different. The leaf-stalk is flattened from side to side, so that the 

 leaves hang vertically. In connection with this it will be observed 

 that while in most leaves the upper and under surfaces are quite un- 

 like, in the Black Poplar, on the contrary, they are very similar. 

 The stomata, or breathing holes, moreover, which in the leaves of 

 most trees are confined to the under-surface, are in this species nearly 

 equally numerous on both. The " Compass Plant " of the American 

 prairies, a yellow Composite not unlike a small Sunflower, is another 

 plant with upright leaves, which, growing in the wide open prairies, 

 tend to poiut north and south, thus exposing both surfaces equally to 

 the light and heat. It was shown by diagrams that this position also 

 affected the internal structure of the leaf. 



In the Yew the leaves are inserted close to one another, and are 

 long and linear; while in the Box they are further apart and broader. 

 In the Scotch Fir the leaves are linear, and 1^ inch long, while in 

 other Pines, as, for instance, the Weymouth, the stem is thicker and 

 the leaves longer. 



In the plants hitherto mentioned, one main consideration a^Dpears 

 to be the securing of as much light as possible ; but in tropical 

 countries the sun is often too powerful, and the leaves, far from 

 courting, avoid the light. The typical Acacias have jnnnate leaves, 

 but in many Australian species the true leaves are replaced by a 

 vertically flattened leaf-stalk. It will be found, however, that the 

 seedlings have leaves of the form typical in the genus. Gradually 

 the leaf becomes smaller and smaller, until nothing is left but the 

 flattened leaf-stalk or phyllode. * In one species the plant throughout 



