Chap. VII] COLD TEMPERATE FOREST FORMATIONS 559 



bright situations do competitors struggle for space. The greater part of 

 the forest soil is occupied by plants which can best develop there. 



The light is too weak for the development of a rich underwood in the 

 summer-forest of the colder zones, the humidity is also too slight to 

 enable plants to climb the trees towards the light. Only in the very 

 moist summer-forest of Japan, where the winters are mild, do some 

 lianes attain dimensions like those in the rain-forest ; elsewhere, the 

 summer-forest does not secure a combination of great heat and great 

 humidity sufficient to enable the young lianes to climb rapidly up to the 

 leaf-canopy, while the desiccating influence of winter would quickly kill 

 their long and delicate stems. In Europe and North America, therefore, 

 lianes occur only in well-lighted woodlands, or at the edge of the forest. 



Epiphytes, the most perfect productions of the struggle for light, are 

 even less developed in the summer-forest than are lianes. One finds here 

 and there in the hollows of old tree-trunks, or by the water-side, herbs 

 and small shrubs, the seeds of which have been carried thither by the 

 wind or by birds l . But epiphytes, that is to say, plants specially adapted 

 to the conditions of life upon the surface of other plants, do not 

 develop from these. The depth of substratum that they demand, and their 

 occurrence only in the vicinity of water, together show that, in order to 

 become epiphytes, such plants require air always saturated with vapour, 

 which at night-time is precipitated as dew, and also require permanently 

 high temperatures to render it possible for them to utilize at all times 

 the scanty but frequently renewed supplies of water of the substratum. 

 Only a few epiphytes have ventured out of the tropical rain- forest into 

 the summer-forest of cold-winter districts, namely, Malaxis japonica in the 

 moist forests of Japan, Polypodium incanum and Tillandsia usneoides in 

 North America, all of them forms that in their homes had become adapted 

 to long periods of drought and therefore able to endure the desiccating 

 influence of winter. Usually only mosses and lichens have found a home 

 on the bark of trunks and branches of trees ; they are organisms that con- 

 tinue to exist for months together in a desiccated or frozen condition, and 

 greedily imbibe atmospheric precipitations over their entire surface. 



The smaller terrestrial plants— shrubs, herbs, mosses — colonize the places 

 within the summer-forest that afford them sufficient light, in which respect 

 their demands vary, and they make as much use as possible of the spring 

 months, during which the illumination renders the conditions to some 

 extent favourable. Most shrubs in such positions become green at an 

 earlier date than do the trees which shade them ; those alone delay the 

 development of their foliage that require, like Cornus sanguinea, only 

 a very moderate amount of light for that purpose. Within this short 



1 Wittrock, op. cit. 



