202 ilow (R^ps pfiER. 



small, and its adhesion to the particles of soil more firm 

 for that reason. Again, no precise boundary can always 

 he drawn between capillary and hydrostatic water, espe- 

 cially in soil having fine i)ores. The terms are neverthe- 

 less useful in conveying an idea of the degrees of wet- 

 ness or moisture in the soil. 



Roots Absorb Capillary or Hygroscopic Water,— It is 

 from capillary or hygroscopic water that the roots of most 

 agricultural plants chiefly draw a supply of this liquid, 

 though not infrequently they send roots into wells and 

 drains. The physical characters of soils that have been 

 already considered suffice to explain how the earth acquires 

 this water ; it here I'emains to notice how the plant is re- 

 lated to it. 



As we have seen (pp. 35-38), the aerial organs appear 

 incapable of taking up either vapor or liquid water from 

 the air to much extent, and even roots continually exhale 

 vapor without absorbing any, or at least without being able 

 to make up the loss which they continually suffer. 



Transpiration of Water through Plants. — It is a most 

 familiar fact that water constantly exhales from the surface 

 of the ])lant. The amount of this exhalation is often very 

 great. Hales, the earliest observer of this yjhenomenon, 

 found that a sunflower whose foliage had 39 square feet 

 of surface, gave off in 24 hours 3 lbs. of water. A cab- 

 bage, whose surface of leaves equaled 19 square feet, ex- 

 haled in the same time very nearly as much. Schleiden 

 found the loss of water from a square foot of grass-sod to 

 be more than 1^ lbs. in 24 hours. Schiibler states that in 

 the same time 1 square foot of pasture-grass exhaled 

 nearly 5^ lbs. of water. In one of Knop's more recent 

 experiments, {Vs. St., VI, 239), a dwarf bean exhaled 

 during 23 days, in September and October, 13 times its 

 weight of water. In another trial a maize-plant transpir- 

 ed 36 times its weight of water, from May 22d to Sept. 

 4th. According to Knop, a grass-plant will exhale its own 



