660 GEOGRAPHICAL AND GEOLOGICAL BOTANY. 



arrested) ; but many survive at a much lower temperature. Every plant 

 has a zero of its own, beyond which it does not thrive. 



M. de Candolle divides plants, in their relation to temperature, into : 1. 

 Macrotherms, or plants requiring much heat, as the plants of intertropical 

 regions ; 2. Mesotherms, plants of the subtropical and warm temperate 

 zones, subject to great heats and never for any length of time to frost ; 



3. Mtiotherms, or plants characteristic of the cool temperate zones ; and 



4. Microtherms, or alpine or arctic plants capable of existing in very cold 

 regions. 



The effect of temperature on plants depends very greatly upon their habit : 

 thus annuals which accomplish their life functions in a few months aro 

 indifferent to the winter's cold if there is heat enough to enable them 

 to ripen their seed before the winter comes on. Other plants are pre- 

 vented from spreading from warm latitudes to colder by deficient summer 

 heat : hence a division of plants into heat lovers (Plaloiherms) and cold 

 fearers (Frigofuges). 



Altitude. An important qualification arises from the existence of high 

 mountains within temperate and tropical latitudes. The temperature of 

 the soil and atmosphere diminishes with the degree of elevation above 

 the surface of the ocean ; and a succession of limits are found upon the 

 sides of high mountains comparable, and in a great degree proportional, 

 to the polar and equatorial limits of plants. Mountains situated in the 

 tropics possess zones of climate which, at successive elevations, resemble 

 horizontal zones situated between the base of the mountains and the 

 poles. 



Drought and Moisture. Moisture and drought are only to be called 

 secondary climatal influences, from the circumstance that they depend in 

 a great measure upon temperature, either directly or indirectly. In the 

 Arctic regions and upon mountain-tops, covered with eternal snow, there 

 is drought from the solidification of the water. Tu temperate and tropical 

 regions the degree of humidity is dependent not merely upon temperature, 

 but upon this combined with the configuration of the surface of the earth 

 and the nature of the soil. Wherever the temperature of the atmosphere 

 is above the freezing-point of water, it takes up aqueous vapour : even 

 ice evaporates in warm air ; and water evaporates in proportion to the 

 temperature to which it is exposed. 



The ocean, especially in warm latitudes, gives up vast quantities of 

 aqueous vapour to the atmosphere, which it delivers in very various ways 

 to the land regions. Islands and coasts in general have a moist climate, 

 while the interior of continents is dry ; but these rules are interfered with 

 by currents or prevailing winds, carrying air loaded with moisture in 

 particular directions. As an example of this, we find the west coast of 

 Europe with a much damper climate than the opposite east coast of 

 America, owing to the Gulf-stream and the winds bringing the moisture 

 of the West-Indian Sea in a north-west current on to our coast. On the 

 contrary, the prevailing winds blow off the west coast of North Africa, 

 and those received on the north-east side come from over dry land, whence 

 a vast tract of land lying in this continent becomes an arid desert. 



There is a great difference also between mountainous and flat countries. 

 The cold upper regions of tropical mountains arrest the prevailing currents 

 of air and precipitate the moisture which they contain. Hence the tracts 



