TEE WORK OF ROOTS 117 



capillary connection with the moist earth below. 

 How a surface layer of loose, dry soil conserves 

 moisture is well seen in Fig. ^^a^ which shows the 

 new growth, during one summer, of two trees side by 

 side under the same conditions, except that in one case 

 the ground was cultivated while in the other it was not. 



The loose, dry layer of earth, known as the surface 

 mulch, must be maintained by cultivating the surface 

 as often as it becomes baked into a crust. The surface 

 mulch not only prevents evaporation but it admits air 

 (which is excluded by the crust) . It prevents the soil 

 from cooling oif quickly at night and keeps it from 

 freezing deeply in winter (snow acts as a mulch) . It 

 also prevents rain from running off the surface. In 

 view of these facts, the advice "Water your garden 

 with the rake" becomes important: i.e., as soon as the 

 water has sunk into the ground go over it with a rake 

 and break up the surface crust so as to foi-m a mulch. 



(What connection is there between evaporation from 

 the soil and the formation of dew ? Invert a small 

 tumbler on the surface of moist soil and leave it over 

 night. Are you able to find any dew on the glass the 

 next morning? Why is dew ordinarily formed only 

 at night and more copiously in absence of wind or 

 clouds?) 



Is all of the rain-water which soaks down through 

 the soil drawn back up again ? How are springs 

 formed? How deep must we go before we find wet 



