236 LIFE OF A TREE, 
totally unfit for cultivation, or at least for the 
cultivation of the same plant. Man, for instance, 
when he reaps the corn-field, carries away a large 
portion of saline and other fertilizing substances 
in the wheat, which he stores in his garner. But 
he does not next year sow the same crop in the 
same field, because he knows he has in a measure 
exhausted it, and must wait until, by the chemical 
influence of the air upon the soil, fresh saline and 
mineral substances are set free. Now in order to 
give back a portion of such matters again to the 
field, what does man do? Precisely what Nature 
does, as will be easily perceived from the fol- 
lowing consideration. The straw of the wheat is 
exposed to moisture and air in the stable; it be- 
gins to rot, is then thrown on the dunghill, and 
is in course of time, when quite rotted and brown 
—now being termed manure—again spread over 
the fields. Thus the various substances it con- 
tained are in due time restored to the soil again, 
with which they become mingled by means of 
ploughing or harrowing. Such, too, is Nature’s 
plan. In Autumn, the now useless leaves are 
strewed on the earth, and in Winter a large 
number of plants die. So soon as this takes 
—— 
