EXHAUSTION OF THE SOIL. 99] 
effects of old age. As they are unable to leave 
the position in which they are found in the soil, 
their only means of obtaining fresh nourishment 
being the elongation of their roots, it is very 
evident when they have consumed all the food of 
an appropriate kind that they can find, they must 
become gradually more and more feeble, and 
will at last die slowly of starvation. It is true, 
the roots are ever on the search for fresh food; 
but then there may be certain difficulties in the 
soil itself which they cannot overcome. Let us 
quote for example the instance given us by our 
Saviour in the parable of the sower, the seed 
sown on a rock, the plant quickly springs up; 
but because there is not sufficient earth and 
room for the roots, it soon withers away and 
dies. So under the surface of the ground 
there may be various obstacles which render 
it impossible for the roots to proceed beyond a 
certain distance; and the inevitable consequence 
to the tree will be, first, that it becomes flower- 
less and fruitless, and finally that it perishes 
altogether. 
In addition to these circumstances, a number 
of causes acting as it is said purely from without, 
