558 ZONES AND REGIONS [Pt. Ill, Sect. II 



Japan, rare in the broad-leaved summer-forest ; they are also small and 

 exhibit but little variety. Epiphytic vegetation is even more poorly repre- 

 sented. The bark of the trees bears only a few small mosses and lichens, 

 and in the deepest shade at most a delicate film of soredia. 



In the thinner woods or near the edge of the forest, wherever light gains 

 admission more readily, the gaps are better filled. This is especially the 

 case near the water-side, where the vivifying influence of light reinforces 

 that of great humidity. Here, the shrubby underwood forms thickets ; 

 ivy, the poison-sumach, and other woody though usually thin-stemmed 

 lianes climb the boles of the trees ; others again are intertwined with the 

 shrubs ; and the bark of the trees is covered with cushions of moss. Yet 

 even with the aid of great humidity and of the heat of summer, which 

 favours rapidity of growth, the luxuriance of rain-forest is not attained, 

 although the summer-forest of Japan, favoured in both these respects, 

 approaches it more closely. The brevity of the warm period, the cold of 

 winter with its desiccating influence, impose narrower limits on the develop- 

 ment of vegetation than in the case of the constantly moist rain-forest. 



The significance of the amount of light that falls upon a forest is shown 

 by the increase of shade-vegetation when the trees of the canopy are more 

 widely separated, or by a comparison between the scanty shade-flora 

 during summer and the relatively luxuriant flora during spring, or between 

 that of the constantly dark coniferous forest and that of the periodically 

 lighted broad-leaved forest. The much greater wealth of vegetation in 

 the shade in the warm zones is attributable, in part, directly to the greater 

 intensity of tropical light, but, in part, also to the fact, established by 

 Wiesner, that the demand for light by plants increases as the supply 

 of heat diminishes, so that one and the same species thrives in deep shade 

 in a warm climate, but only in bright situations in a temperate climate. 



Wiesner has determined the extremely important lowest intensity of light for 

 several deciduous broad-leaved trees at Vienna, from the middle of May until the 

 middle of July, and gives the following figures, among others, for closed woods : — 



L (min.). I (max.). 



Fagus sylvatica • • to .... 0-02 1 



Aesculus Hippocastanum st • • • • 0-023 



Quercus pedunculata o\ .... 0-050 



Fraxinus excelsior ^l 0-224 



Hence, in the summer-forest, as well as in the rain-forest, vegetation 

 remains under the spell of illumination. Yet the struggle for light in the 

 summer-forest does not generally consist, as it does in the rain-forest, in 

 an internecine contest for possession between strong organisms, in which 

 even tall trees are vanquished by small plants, but is one solely of adapta- 

 tions to the unfavourable conditions of light. Only in specially favoured 



