THE ATMOSPHERE 77 
absorption resulting from it. Plants which grow in windy 
situations therefore exhibit xerophytic characters, or they 
soon perish, and where the wind is incessant and fre- 
quently violent, tall growth fails altogether, giving place 
to a low or dwarfed vegetation. This is well seen on 
wind-swept downs by the sea. On Beachy Head, for 
example, trees are absent, and the ground is covered with 
short, thick grass ; what bushes there are are low, stunted, 
and dense, while the herbaceous weeds scarcely rise above 
the level of the herbage. It is curious to observe how 
plants which in the sheltered hollows and dips grow to 
their natural size of several feet, sink on the exposed 
downs to an inch or two, bearing a few leaves, and 
perhaps a single flower. But the flower is not reduced 
in any way; if anything, its colours are brighter than 
usual. 
Where trees occur in windy places, they are bent away 
from the wind, and the branches and foliage develop en- 
tirely on the sheltered side. This is because the buds on 
the exposed side do not develop, or, if they do, the young 
shoots arising from them are killed by the drying action 
of the wind. Sometimes a curious phenomenon is wit- 
nessed. Springing from a broad crown of foliage rises a 
miniature forest of short, erect, dead twigs. These were 
shoots which, in their upward growth, just touched a 
zone where the wind was stronger than their powers 
of endurance; they ceased to grow, dried up, and 
perished. 
Atmospheric Equilibrium.—Before leaving the subject 
of the air, we will refer to a phenomenon which at first 
sight is not altogether easy of explanation. The world 
is teeming with life, animal and vegetable. Gases are 
continually being withdrawn from the air and discharged. 
into it, and yet it is a matter of common knowledge that 
the constitution of the atmosphere remains practically 
constant at all times and in all places. How is this equili- 
brium brought about and maintained ? We will en- 
deavour to answer the question by preparing a balance- 
sheet of the losses and gains of the atmosphere : 
