322 INTRODUCTION TO BOTANY 



People now know that it is of the greatest importance to 

 till the soil during droughts, in order that it may not lose 

 its moisture. An illustration will help in understanding this. 

 If two pieces of loaf sugar are placed one upon the other, the 

 lower one held in the thumb and finger and the other left 

 lying loosely upon the first and not touching the fingers at 

 all, and if the lower one is then placed in contact with water, 

 two important facts are shown: The lower piece takes up 

 water freely, but the upper one, though lying upon the lower 

 wet piece, becomes wet only after a long time. Close con- 

 nection between the solid particles is necessary for the rapid 

 upward passage of the water. 



When soils are compact, moisture from the deeper portions 

 passes upward freely, as in the lower lump of sugar, and 

 evaporates into the air. If, however, the surface is kept loose 

 and finely pulverized, so that the particles are less closely con- 

 nected, moisture does not readily pass through it, and there is 

 not so much loss from evaporation. The roots of plants, being 

 more deeply placed, are in contact with the moist soil from 

 which a supply of water may be secured. The depth to which 

 roots are known to go in regions where the water is found 

 only at great depth is discussed in section 22. 



It has been generally supposed that tilling the soil serves 

 the twofold purpose of regulating the moisture supply for 

 growing plants and of preventing the growth of weeds. Ex- 

 periments upon the cultivation of corn seem to show, however, 

 that if all the weeds are removed without disturbing the soil, 

 the yield is practically the same as when the soil is tilled. 1 



303. Dry farming and irrigation. In regions with a very 

 scanty rainfall frequent tillage of the surface seems to enable 

 the soil to hold most of the water that falls, and in this way a 

 crop may be grown, sometimes every season and sometimes every 

 second season. Evidently much work is necessary to enable 

 the soil to accumulate enough moisture to supply the needs of 



1 "The Weed Factor in the Cultivation of Corn," Bulletin 257, Bureau 

 of Plant Industry, U.S. Dept. Agr., 1912. 



