eee FLOWERING PLANTS 
the temperate zone, which in its turn differs from that of the 
arctic regions. This is due to the difference in temperature, 
which, other things being constant, varies with the latitude. — 
The altitude, or the height above the sea-level, may neutralize: 
the effect of latitude. This partly accounts for a vegetation 
similar to that of temperate regions being found on tropical 
mountains of a certain elevation. At the same time, this 1s 
not the whole of the explanation. The fact of temperature 
varying with latitude accounts for the possibility of alpine 
plants being able to live in tropical regions, but it does not 
explain their origin in those latitudes. To understand that, 
some knowledge of the climatic changes in past ages is neces- 
sary, and is more fully discussed later (Chapter III). 
The rainfall of a country and the amount of water present 
in any form, as dew, snow, mist, fog, or sleet, is of great 
importance. In certain parts of the tropics, where there are 
two seasons—the dry and the rainy—and where the dew at 
nights is heavy even in the dry season, the vegetation is 
luxuriant. In the same latitudes, where, instead of an abun- 
dant rainfall, there is practically no rain, deserts will occur. 
In connection with the amount of moisture of a district, the 
prevailing wind should be considered. Winds that come over 
an ocean are more laden with moisture than those which have 
swept a continent. The physical nature of the soil, whether 
‘porous or not, modifies the effect of rainfall, and consequently 
the amount of the water-supply of the plant. Limestones— 
such as chalk—sand, and gravel, are very porous, and do not 
retain much water. The vegetation of such a soil as sand is 
far more limited than that ot clay, which does hold water, for 
it is only plants which can protect themselves from too rapid 
transpiration that will be able to adapt themselves to a very 
porous soil. The presence of humus (Lat. humus, soil), or 
decaying vegetable matter, is essential. to the production of 
good soil, and has several important functions. It increases 
the amount of water which sandy soils can retain, it tends to 
preserve the porous nature of stiff clays, and altogether to 
promote the friable condition of the soil. 
