INTRODUCTION 29 



The leaves may be arranged on the stems in layers 

 or horizontally, allowing light to penetrate between 

 as in the Lime. Or the leaves may grow out on long 

 stalks at right angles, and in such cases they form a 

 pattern or mosaic so that each leaf more or less faces 

 the light. They are usually opposite, the lower leaves 

 larger forming an outer ring, the inner smaller fitting 

 in between. With these arrangements the best 

 position for the stomata in trees is below, but when 

 the leaves are pendent as in the Aspen there are 

 stomata both sides. In some plants they are on 

 both sides, especially where, as in grasses and other 

 plants, the leaves are erect, when the leaves are gene- 

 rally narrow and linear, or like those of aquatic 

 plants. In hot countries and on dry soils the leaves 

 are reduced in area and otherwise adapted to physio- 

 logical needs. 



Leaves may be evergreen or deciduous. In the 

 former they are glossy and smooth, so that snow 

 does not cling to them and cause the boughs to 

 snap. If long-lived they are leathery, a protection 

 against animals. In deciduous trees the leaves and 

 stalks are separated from the stem or twig by a 

 corky layer, which protects the leaf scar, and the 

 leaf fall is coincident with the adaptation of the 

 tree to dry conditions or rather physiological drought 

 of winter cold. 



accessory devices, as pitcher leaves, are all also related to the con- 

 ducting of water to the root-hairs, either to the centre or away 

 from it. 



