THi: WORK OF ROOTS 



115 



shorter arm, it shows that some of the water which 

 has run down through the soil is being drawn up again. 

 In which tube does this take place more rapidly ? Is 

 it hastened by putting the tubes in the sun? Explain. 

 It often happens that a bed of gravel lies from three 

 to six feet below the surface of the soil ; how does this 

 affect the upward move- 

 ment of water in the soil ? 

 Place a little gravel in the 

 tube and note the effect. 



To retain the moisture 

 in the soil, the farmer re- 

 sorts to cultivation and 

 mulching, i. e., placing 

 on the surface of the soil 

 something (straw, dead 

 leaves, stable manure, 

 etc.) to prevent evapora- 

 tion. 



Take four student-lamp chimneys (Fig. 88), cork 

 them securely at the bottom and fill them with good 

 moist soil (which should be previously well mixed so 

 as to be uniform throughout) . On the top of the soil 

 in one, place a layer of dry sawdust four inches deep 

 (a) ; in another place a similar layer of sifted dry 

 soil not packed down (/>) ; in another plant Wheat 

 (c), and leave the last as a control {d) . Weigh 

 each of them, and repeat the weighing at intervals 



88. Four lamp-chiiinieys filled with soil for 

 the purpose of studying the rate of evap- 

 oration from the surface: (a) surface 

 covered with a mulch of sawdust; (ft) 

 surface tilled; (c)surface planted; {d) 

 control. 



