98 THE WORLD OF LIFE CHAP. 



over the whole world, an unexpected amount of agreement 

 in the numbers of the flowering plants, but always showing 

 a moderate increase from the colder to the warmer parts of 

 the earth. 



Differences of Temperate and Tropical Vegetation 



One of the chief differences between the floras of the 

 colder and of the warmer parts of the earth (already referred 

 to) is the greater prevalence in the former of gregarious 

 plants. Towards the northern limit of vegetation we find 

 continuous forests of pines or firs, the same species often 

 extending for hundreds or even thousands of miles ; while 

 woods of birches extend even farther north almost up to the 

 limits of perpetual snow, and in this case a single species 

 our common birch extends entirely across northern Europe 

 and Asia, with allied species in North America. Farther 

 south, forests of beeches, oaks, chestnuts, etc., are common, 

 but seldom covering such large areas, being dependent on 

 conditions of soil as well as of climate ; while in the warmer 

 parts of the temperate zone the forests are often made up of 

 a great variety of trees, though never so completely inter- 

 mingled as in the typical areas of the tropics. 



Another, and perhaps more important character of the 

 tropical flora, is the large number of distinct types of vegeta- 

 tion which are almost or quite peculiar to the warmest and 

 most equable regions of the earth. This is indicated by the 

 fact that about one-fourth of the natural orders of plants are 

 either exclusively tropical or very nearly so, and that they 

 comprise such remarkable forms as the epiphytic Orchids, 

 the Bromelias, the Palms, the Pitcher-plants, Bananas, Bread- 

 fruits, the Coffea and Cinchona trees, and hundreds of others 

 almost unknown except to botanists. 



But the most striking feature of all is the wonderful 

 adaptations by which every well-marked place or station is 

 occupied by peculiar groups of plants. The epiphytes above 

 referred to plants which live upon trees, upon the trunks 

 or branches, and especially in the forks, where they can root 

 and establish themselves, not as parasites by sending their 

 roots into the living tissues of the tree, but solely getting 

 nourishment from the rain-water that trickles down the bark 



