108 ANNUAL REPORT OF NEW-YORK 



(except in excessive wet or drought) maintains the moistness 

 which is salutary to most of our cultivated plants. 



What part the capillarity of the soil plays in the nutrition of 

 the plant may now be noticed in detail. 



If a wick be put into a lamp containing oil, the oil by capillary 

 action gradually permeates its whole length, that which is above 

 as well as that below the surface of the liquid. When the lamp 

 is set burning, the oil at the flame is consumed, and as each parti- 

 cle disappears, its place is supplied by a new one, until the lamp 

 is empty or the flame extinguished. 



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 place of the combustion of the 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 recur- 

 rence of a dry atmosphere with sunshine and wind, the surface of 

 the soil rapidly dries; but as each particle of water escapes, (by 

 evaporation) into the atmosphere, its place is supplied (by capil- 

 larity) from the stores below. The ascending 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 dryer 

 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 lamp wick, no more evaporation into the air can occur, 

 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 subterranean fountains yield a meagre supply, 

 then capillarity will aid gravity in its downward distribution. 

 The water of the soil holds in solution the food of the plant — 

 those portions at least which are absorbed by the roots. From 

 the leaves of growing plants there is perpetually going on an 

 enormous evaporation. Calculations founded on experiments of 

 Hales and Saussure demonstrate that from an acre of sunfloAvers, 

 each plant occupying four square feet of ground, there occurs 

 during four months growth, the evaporation of four and a half 



