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. 
