64 Principles of Plant Culture. 
the rooted slip is placed in other oil-covered water that has 
been exhausted of its oxygen by boiling, the roots will soon 
die. 
The copious forma- 
tion of root-bairs (101) 
that reach out into the 
moist atmospbere of 
\ ihe seed-tester (39), 
and that so often fill 
the soil cavities with a 
delicate, cottony down, 
is furtber proof that 
roots search for air as 
well as water. The to- 
tal absence of live 
rootlets in the puddled 
clods of badly tilled 
fields, shows that roots 
will not penetrate soil 
from which the air has 
Fie. 25. Slips of Tradescantia in water con- 
taining oxygen (left glass) and in water contain- 
ing no oxygen (right glass). From nature. been expelled by undue 
compression while wet. Plants in over-watered greenhouse 
pots sometimes send rootlets into the air above the soil to 
secure the oxygen from which their roots have been de- 
prived. 
91. The Ideal Soil for Land Plants must contain 
enough plant food and water to fully supply the plants, and 
yet be so porous that air can circulate through it, and come 
in contact with the roots. Each particle of such a soil is 
surrounded with a thin film of water, while between the par- 
ticles are spaces connected with each other, and filled with 
