18 PHILOSOPHY OF ZOOLOGY. 
life would often be interrupted at its commencement, and 
few individuals would reach the period of maturity. But 
the same faculty which directs the first movements of the 
organized body to obtain a supply of food, likewise assists 
in removing or counteracting opposing difficulties. 
B. Obviates Difficulties —When the roots of a plant, 
spreading in search of nourishment, meet with interruption 
in their course, they do not cease to grow, but either at- 
tempt to penetrate the opposing body, or to avoid it, by 
changing their course. Thus, I have repeatedly seen the 
creeping root of the T'riticum repens, or Couch-grass, 
which had pierced through a potato that had obstructed 
its course: and every one knows, that the roots of a tree 
will pass under a stone, wall or ditch, and rise again on the 
opposite side, and proceed in their original direction. 
The Shipworm, T'eredo navalis, an animal which perfo- 
rates wood, in order to form for itself a habitation, and 
which is well known as most destructive to the timbers of 
ships and harbours, in general, lengthens its cell by boring 
in the direction of the fibres of the plank. Should it hap- 
pen to meet with the shell of another animal of the same 
kind, or a knot in the wood, which it is unable to penetrate, 
it changes its course, so as to avoid the obstacle, either by 
a slight curvature, or by reversing its original direction. 
When two trees of the same kind are planted, the one 
in a sheltered, and the other in an exposed situation, we 
witness the display of this faculty, now under considera- 
tion, in a very remarkable degree. 'The former pushes 
forth its roots in all directions, more especially where there 
is the greatest supply of nourishment, and the highest tem- 
perature; while the latter, which, were it to act in the same 
manner, would be speedily overturned, multiplies its roots 
in the direction of the strongest blasts, and these, acting like 
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