l] WHAT IS PLANT GEOGRAPHY ? II 



widely more than 762 mm. (30 inches). There is a marked difference 

 between winter and summer light-climates and temperatures, and 

 often, precipitation. The vegetation tends to be fairly luxuriant at 

 least in favourable situations, with trees and shrubs widely dominat- 

 ing but herbaceous plants usually exceeding them in number and 

 variety. Most areas having a ' Mediterranean ' type of climate, 

 with hot and dry summers but with other seasons that are damp 

 and not too cold for plant growth, are included here, their vegetation 

 being often dominated by leathery-leafed shrubs but including 

 many bulbous and ephemeral herbs. The main vegetation-types of 

 temperate and adjacent lands are dealt with in Chapter XII. 



The climates of polar lands and high altitudes are mostly rigorous, 

 with the mean of the warmest month usually below 10° C. Pre- 

 cipitation is mostly in the form of snow and widely less than 254 mm. 

 (10 inches) per annum, though owing to the prevailingly low tem- 

 peratures the relative humidity may be high and the evaporating 

 power low. There are wide seasonal fluctuations in most polar 

 regions and wide diurnal ones in most alpine areas. In the higher 

 latitudes there is continuous light in summer and darkness in 

 winter. The vegetation is mostly low and scant — of dwarf shrubs, 

 herbs (including many of grass habit), Lichens and Mosses. The 

 main vegetational types of polar lands and high altitudes are dealt 

 with in Chapter XIII. 



The climates of tropical and adjacent lands are warm and widely 

 humid, with the mean temperature of the coldest month usually 

 above 17*8'' C. (64'' F.) and the rainfall often heavy {e.g. 200-400 cm.). 

 Frost and snow are usually unknown, the conditions being torrid 

 and widely equable, with often little or no seasonal variation. The 

 vegetation ranges from the world's most luxuriant rain forest to 

 various scrub, grassland, and desert communities as the available 

 water decreases. Most ' monsoon ' areas of alternating wet and dry 

 seasons, commonly dominated by deciduous trees and shrubs which 

 lose their leaves to conserve water during dry periods, are included 

 here. The main vegetational types of tropical and adjacent lands 

 are dealt with in Chapter XIV. 



It should be noted that the distinction between even ' oceanic ' 

 (' maritime ', or ' insular ') and uneven ' continental ' climates is 

 largely one of degree, being irrespective of latitude or temperature- 

 relationships and consequently found in all of the above three primary 

 climatic groupings. In general the oceanic extreme occurs on land 

 where the prevailing winds come off the sea and are consequently 



