NATURE OF PLANTS 55 



time and the soil is properly aerated. Injudicious sprinkling of 

 lawns, gardens, or potted plants will result in the formation of a 

 compact surface crust. This acts exactly like a clay soil and 

 draws the water away from the region of the roots to the surface 

 of the soil where it is lost by evaporation. Potted plants are 

 best watered by standing the jar in the water until the soil is 

 wet. Further watering is not required until the jar sounds 

 hollow when tapped. So gardens and lawns should be thor- 

 oughly soaked during the night. The ordinary sprinkling only 

 moistens the surface and leaves the soil in a worse condition for 

 holding the moisture than before, because on drying it bakes 

 and the hard crust draws up the water to the surface where the 

 dry air readily evaporates it. A light shower in the summer time 

 may produce the same injurious effect. On the other hand when 

 the surface of the soil is loose and air dry, so that the capillary 

 water' does not extend from the surface to the deeper strata, 

 then a shower may do harm in quite another way. By supplying 

 the upper particles with films of water it reestablishes capillary 

 connection with the lower lying particles so that the film water 

 now begins to move again towards the surface of the soil. This 

 explains why the top soil should be harrowed and hoed during 

 periods .of inadequate rainfall. The loosely arranged particles 

 quickly become dry and the capillary films leading to the water 

 supply below are thus broken at the surface and further loss of 

 water through evaporation is in part checked. A mulch of straw 

 and leaves upon the surface of a soil has the same result as 

 harrowing. Sometimes it is desirable to have a compact surface 

 layer of soil, as for example in sowing a field to grain or grass. 

 This is accomplished by rolling the field, which not only gives a 

 smootli surface to facilitate the cutting of the crop but also makes 

 a compact stratum at the surface which draws the moisture up 

 to the grains, causing them to grow. Otherwise the surface of 

 the soil containing the grains would be so dried out by the winds 

 often in a single day, as to prevent their sprouting. 



23. Extent of the Root Surface. — It may seem surprising that 

 the roots are able to take up the large volumes of water that are 

 lost daily through transpiration. The spread of the root, how- 



