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 



