20 VEGETABLE PHYSIOLOGY 
to the wind, often so completely as to place their long axes 
parallel to the direction in which it is blowing. 
Another reason for this continued subdivision of the 
plant-body is found in its relation to the absorption from 
the soil which we have found associated with the root. 
The latter is continually absorbing the water of the soil; 
after separating from such water the mineral constituents 
it contains, a very large part of the water is evaporated, 
and so passes to the exterior again. ‘To favour such evapora- 
tion it is advantageous that the ratio between surface and 
bulk shall be a large one, and so the great subdivision of 
the subaerial part of the plant is concerned in solving 
the problem of its nourishment. 
Indirectly the composition of the subaerial part of 
the plant has an application to a danger to which the 
underground region is exposed. ‘The pressure of the wind 
upon an unyielding surface in the air would be attended 
by great danger to the anchoring root, which might be 
violently pulled from the ground by the leverage exerted 
by such pressure. The great subdivision of the shoot 
system and the flexibility of its ultimate twigs minimises 
this danger, but even as it is it is not unusual after a tempest 
to notice the uprooting of trees of quite considerable girth. 
The distribution of the water of rainstorms calls for 
particular arrangements of the parts of the shoot. ‘The 
water can be led either towards or away from the centre 
of the plant. Should the root system be one which spreads 
considerably and extends to long distances below the 
surface of the soil, it is of great Importance that the rainfall 
collected on the central mass of the shoot system shall be 
distributed widely so as to reach the extremities of the 
roots, watering thus a large area of ground. This is brought 
about by suitable positions taken by the flattened parts and 
the grooving of certain of the cylindrical parts of the shoot 
system, causing the water to be conducted outwards. If 
the root system consists, on the other hand, of a strong main 
root with comparatively few branches this arrangement 
