60 



PRINCIPLES OF PLANT CULTURE 



agitate the rest of the water vigorously to impregnate it 

 again with oxygen, and pour some of this into the sec- 

 ond tumbler. Set both tumblers in a light, warm place. 

 In a few days roots will start freely from the slip in the 

 tumbler in which the water has access to the air, but not 



in the other (Fig. 27). 

 If now the rooted cut- 

 ting is placed in oil- 

 covered water that 

 has been exhausted of 

 its oxygen by boiling, 

 the roots will soon die. 

 The copious forma- 

 tion of root-hairs 

 (100) that reach out 

 into the moist atmos- 

 phere of the seed- 

 tester (38), and that 

 so often fills the soil 

 cavities with a deli- 

 cate, cottony down, 

 is further proof that 

 roots search for air 

 as well as water. The 

 total absence of live 

 rootlets in the pud- 

 dled clods of badly- 

 tilled fields shows that roots will not penetrate soil from 

 which the air has been expelled by undue compression 

 while wet. Plants in overwatered greenhouse pots 

 sometimes send rootlets hito the air above the soil to 

 secure the oxygen from which their roots have been de- 

 prived. 



Fie. 27. — Slips of tradescantia in water 

 containing oxygen (right glass) and in 

 water containing no oxygen (left glass). 

 From nature. 



