330 MESOPHYTES SECT, xvi 



with climate, and is exhibited most frequently in temperate and cold 

 regions where there is a winter, but also within the tropics where the dry 

 season is protracted. Tropical forests have already been discussed in 

 connexion with xerophytes ; in them the old leaves are often stiff or hairy. 

 In mesophytic deciduous forests, on the contrary, the leaves are thin, flexible, 

 and relatively translucent, possessed of a thin-walled epidermis, dorsi- 

 ventral in structure, and are often plastic (as in Fagus) in relation to 

 external conditions. They usually arrange their blades perpendicular to the 

 strongest diffuse light. Their shapes are manifold. There are undivided, 

 divided, and compound leaves ; but their division is not so complete nor 

 into so many leaflets as in the case of species belonging to tropical rain- 

 forest. 



There is therefore a season of foliation and one of defoliation. At the 

 former time one sees only the young, usually fresh-green, shoots ; but in 

 the tropics, reddish tints due to anthocyan also show themselves, and 

 may even occur in Central Europe, for instance, on species of Quercus 

 and Acer. The foliage gradually assumes a darker tint of green as summer 

 progresses ; before leaf-fall, yellow and red colours appear, partly because 

 the chlorophyll is decolorized, as in yellow leaves, and partly because 

 anthocyan is produced, as in red leaves (which are displayed in their full 

 glory by North American trees). 



Leaf-fall is usually associated with the commencement of the cold 

 season ; one and the same species may lengthen or abbreviate its vegeta- 

 tive season according to the local climate. The more proximate cause 

 of the phenomenon must probably be sought in the desiccation that is 

 threatened by the cold soil ; the causes of leaf-fall are certainly the same 

 when it is evoked by cold or by direct drought. 



During the resting season protection against intense transpiration is 

 provided by bud-scales clothing the youngest parts of the shoots, and by 

 cork investing older parts. 



Mesophytic deciduous trees often have a rich system of branches with 

 many small twigs ; nearly all the buds, excepting those near the base of 

 the year's shoot, develop into branches, but conditions of illumination 

 may affect the result. In this way there arises a canopy of leaves more 

 continuous than is wont in a tropical tree. 



Compared with evergreen trees deciduous ones do not live under 

 conditions so favourable to existence, since a large part of their lives is 

 passed in inactivity : they rarely attain the gigantic dimensions reached 

 by evergreen trees in tropical rain-forest. 



In mesophytic forests of temperate countries a leading part is played by 

 the Amentiferae, also by Fraxinus, Acer, Tilia, Populus, and Ulmus ; 

 while in warmer lands many additional trees are gradually added to these. 

 In the forests of North America and Eastern Asia numerous other genera 

 occur. 1 



In the dicotylous forests of northern Europe the trees are mostly wind- 

 pollinated, and bear flowers very early in the year, before or during the 

 act of foliation ; the flowers hibernate inside buds or naked. Some 

 southern forms, lime-trees for instance, do not blossom before summer, 

 and are insect-pollinated. 



In dicotylous forest there are at least two, and commonly several, storeys 

 1 See p. 335- 



