178 HOW cuors fked. 



Sonietliuig quite analogous occurs in the soil, by wliicli 

 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 Avick, and the matter stands 

 thus : Let us suppose dew or rain to have saturated the 

 ground with moisture fur some depth. On recurrence of 

 a dry atmospliere with sunshine and wind, the surface of 

 the soil rapidly dries ; but as each particle of Avater es- 

 capes (by evaporation) into the atmosphei-e, 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, Tiie movement proceeds 

 in this way so long as the surface is drier than the deeper 

 soil. When, by rain or otlierwise, the surf^ice is saturated, 

 it is like letting a thin stream of oil run upon the apex of 

 the lamp-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 cf 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 wliich 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 de})th, and thus are kept 

 constantly changing their place with the Avater, which is 

 the vehicle of their distribution. In regions Avhere rain 

 falls periodically or not at all, this upward flow of the soil- 



