THE WORK OF SOOTS 



95 



above ground into the light and air arid its root down 

 into the soil the better will be its chances of living 

 through this period of its life, which is more critical 

 and more beset with dangers than any other. We can 

 also see that in this connection gravity is the best guide 

 the plant could have, since it is more constant and 

 uniform than any other the only one, in fact, that is 

 absolutely constant. 



What happens if the root in its downward course 

 meets with a dry region? When seeds are grown in 

 damp sawdust, in a box provided with a bottom of wire 

 netting, as described on page 90, how do the roots be- 

 have as they grow down through the netting into the 

 dry air ? Often we see them turn upward, as in Fig. 

 77. Is this be- 

 cause they try to 

 avoid air, light, 

 etc., or are they 

 attracted by 

 something in the 

 sawdust? We 

 may test this by 

 arranging the ex- 

 periment as fol- 

 lows: Fill a tumbler one-third full of water and tie a 

 piece of cheese-cloth over the top; on this put damp 

 sawdust, and in the sawdust place several soaked seeds; 

 invert another tumbler over the whole, as shown in 



77. Arrangement for testing the effect of moisture on 

 the direction of growth of roots. 



