178 HOW CROPS FEED. 
Something quite analogous occurs in the soil, by which 
the plant (corresponding to the flame in our illustration) is 
fed. The soil is at once lamp and wick, and the water of 
the soil represents the oil. Let evaporation of water from 
the surface of the soil or of the plant take the place of 
the combustion of oil from a wick, and the matter stands 
thus: Let us suppose dew or rain to have saturated the 
ground with moisture for some depth. On recurrence of 
a dry atmosphere with sunshine and wind, the surface of 
the soil rapidly dries; but as each particle of water es- 
capes (by evaporation) into the atmosphere, its place is 
supplied (by capillarity) from the stores below. The as- 
cending water brings along with it the soluble matters of 
the soil, and thus the roots of plants are situated in a 
stream of their appropriate food. The movement proceeds 
in this way so long as the surface is drier than the deeper 
soil, When, by rain or otherwise, the surface is saturated, 
it is like letting a thin stream of oil run upon the apex of 
the Jamp-wick—no more evaporation into the air can oc- 
cur, and consequently there is no longer any ascent of 
water; on the contrary, the water, by its own weight, 
penetrates the soil, and if the underlying ground be not 
saturated with moisture, as can happen where the subter- 
ranean fountains yield a meagre supply, then capillarity 
will aid gravity in its downward distribution. 
It is certain that a portion of the mineral matters, and, 
perhaps, also some organic bodies which feed the plant, 
are more or less freely dissolved in the water of the soil. 
So long as evaporation goes on from the surface, so long 
there is a constant upward flow of these matters. Those 
portions which do not enter vegetation accumulate on or 
near the surface of the ground; when a rain falls, they are 
washed down again to a certain depth, and thus are kept 
constantly changing their place with the water, which is 
the vehicle of their distribution. In regions where rain 
falls periodically or not at all, this upward flow of the soil- 
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