10 BRITISH PLANTS 



The effect of solar heat upon climate may be considered 

 in two relations : 



1. Latitude. — ^As we recede from the Equator to the 

 Poles, the sun's rays reach the earth in an increasingly 

 slanting direction. In consequence of this, the heat 

 received from the sun gradually diminishes in effect, 

 owing to the spherical shape of the earth's surface, and 

 the losses sustained by the rays having to pass through 

 increasing thicknesses of air. The Tropics occupy a belt 

 about the Equator, and within this belt the sun is 

 directly overhead twice every year. Here, therefore, 

 there is no alternation of seasons corresponding to our 

 idea of summer and winter, and consequently there is no 

 such break in the period of vegetative activity as that 

 which is so familiar to us — when flowers are nowhere to 

 be seen, when seeds lie dormant, and trees have discarded 

 their summer foliage. 



In the Tropics the " winter " temperature is but a few 

 degrees lower than the " summer " temperature, and 

 the vegetation is always green. There may, however, 

 be another kind of seasonal alternation, depending, not, 

 as summer and winter, upon the variations of heat re- 

 ceived from the sun, but upon the supply of water 

 received in the form of rain — that is, wet and dry seasons 

 may alternate. In some parts of the Tropics this alterna- 

 tion occurs with great regularity, and if the dry season is 

 long and very pronounced, a break in the vegetation may 

 be produced, which is very similar in its effects to that 

 which occurs in winter in northern latitudes — e.g., in 

 the Caatinga forests of the Brazilian plateaux. In the 

 Tropics, therefore, dearth of water has the same effect in 

 arresting the activity of the vegetation as cold in northern 

 winters. 



In regions outside the Tropics, a regular alternation 

 of seasons occurs, and, in consequence, that break in the 

 vegetation known as the winter-rest. In proportion as 

 we recede from the Tropics, the winters increase in dura- 

 tion at the expense of summer, and not only do the 

 summers become shorter, but the sun's power during 

 the daytime becomes less effective, the only com- 

 pensation for this being the increasing length of the 

 individual day as we approach the northern limits of 

 vegetation. 



