223 



sufficient for the germination of the seed and subsequent growth 



of a particular species, it may not be sufficiently high to enable 



the plant to produce fertile seeds, and hence, its distribution by 



seed being prevented, its chances of naturally, establishing itself 



are small. The facts that luxuriant vegetation is characteristic 



of the hottest areas in the tropics, provided there4s sufficient 



moisture, and that good forests exist in the coldest known 



spots of the earth, i.e. in parts of Siberia, suffice to show 



that other factors besides temperature are responsible 



for the distribution of plants, and for the absence of trees in 



parts of the tropics and in the arctic zone. Since a similar Low Tem ~ 



reduction in temperature may be caused, not only by increas- U^T U! to G 



ing latitude, but also by increasing elevation above sea-level, increasing 



we should expect, on ascending mountains, to find charac- Latitude but 



teristic zones, or regions, of vegetation resembling those met Elevation 



with when proceeding from the equator towards the poles, and, above Sea 



to a great extent, this is the case. It must, however, be re- l*vel. 



membered that, although temperature is often the dominant 



factor influencing the distribution of plants in both cases, on 



mountains the increasing rarefaction of the air has an important 



effect on the climate and therefore on the vegetation, which 



is not the case in the lowlands. 



Under temperature, also, we must not only consider the importance 

 temperature of the atmosphere but also that of the soil, seeing peratureTof 

 that, when the soil temperature falls below a certain point, the the Soil, 

 roots are no longer able to absorb the necessary supplies of 

 water and salts from the soil. If, while the roots are thus 

 inactive, the leaves are actively transpiring moisture under the 

 action of bright sunlight and dry air, the plant must suffer 

 from desiccation, as has already been pointed out in Part V 

 above, see page 211. Shallow-rooted, low plants, such as 

 grasses, on the whole probably suffer less in this respect than 

 do trees. 



In the case of the former, as the temperature rises and 

 transpiration increases, the roots situated in the superficial layers 

 of soil are soon warmed sufficiently to enable them to make good 

 the loss, while the tree roots situated in the deeper layers of cold 

 soil are unable to do so. Moreover low plants, with their 

 serial portions situated in the calm, damp, lower layers of the 

 atmosphere, do not transpire so actively as do the crowns of 

 trees fully exposed to the action of drying winds. This factor 

 is of great importance in high mountains where, owing to 

 the short vegetative season, many trees find it necessary to 

 remain in leaf throughout the year, their roots being in winter 



