The Nutrition of Plants. 163 



are actively absorbing water from the soil during the time, it only requires 

 that evaporation shall be less than the supply furnished for the revival of the 

 plant. When the loss of water above is greater than the amount absorbed — 

 however great the latter — wilting must occur ; but revival ensues as soon as 

 the evaporation is sufficiently checked to allow an accumulation of the fluid 

 absorbed by the roots. Sprinkling the leaves simply reduces the evaporation 

 from them, and permits the activity of the roots to sufficiently furnish the 

 tissues with water. It is, however, a fact that badly wilted leaves do absorb 

 some moisture from a saturated atmosphere, but turgid leaves do not, and 

 practically, in the former case, the amount so obtained is scarcely worth con- 

 sidering. We may say that all the water obtained by plants, for whatever 

 use, is furnished from the soil through the roots. 



One of the most remarkable things about this is, that land plants are capa- 

 ble of extracting from soil which seems dry, sufficient water for their use, 

 not only as already mentioned, but to provide also for the marvelous quan- 

 tity constantly exhaled from green leaves— amounting on an average to one 

 and a quarter ounces for each square foot of leaf surface in favorable 

 weather during twelve hours of daytime. As there is on a large forest tree 

 200,000 square feet of leaf surface, the amount sent ofT from its foliage, each 

 sunshiny summer day, reaches the astounding total of seven and three- 

 fourths tons, or nearly fifty barrels of forty gallons each! With ten such 

 trees to the acre, we shall have 500 barrels given off from this area. Experi- 

 ments have shown that an acre of beets throw into the air each favorable 

 day about eighty barrels of water, and this may be taken as a low average 

 for fields of cultivated plants. Maize and sugar cane must give off very 

 much more. Taking this eighty barrels as a basis, think of 51,200 barrels of 

 water pumped from the soil into the hungry air for every square mile o 

 land each fair summer's day— 2,816.000,000 barrels from Illinois in the same 

 time! This represents something over one-eighth of an inch deep over the 

 whole surface. If the plant does owe much of its substance to water, it is 

 amazingly prodigal of the precious liquid! It however must be borne in 

 mind that the figures mentioned are for weather specially favorable for 

 evaporation. During the night and in cloudy days little is exhaled. A clear 

 sun, a high temperature and a brisk breeze greatly promote the transpiration 

 of water from green leaves. 



To obtain their enormous supplies of water from the soil, plants are ex- 

 cellently equipped, in the great development of their root systems, and the 

 clothing of their younger portions by myriads of exceedingly slender epi- 

 dermal cells, called root hairs. Taking the latter into account, it is probable 

 that the root surface of a tree is fully equal to that of the stem and leaves. 

 If a large forest tree has anything like five acres of absorbing root surface, 

 it is not so astonishing, after all, that fifty barrels of water per day can be 

 supplied to the plant. The extent of roots is far greater than usually sup 

 posed. On our Illinois prairie soil, blue grass in pastures sends down roots 

 in considerable numbers five to six feet; clover often twice that depth. 



