The Foot and the Soil. 63 
pumpkin vine and tomato plant above mentioned, proximity 
to the soil furnishes a moist atmosphere. A corn-stalk 
pegged down to the ground for some distance will usually 
root at all the joints of the stem in contact with the soil. A 
potato plant grown under a bell glass, where the air is nearly 
saturated, will put out roots at any joint of the stem. In 
parts of the tropics where the air is very moist, certain 
plants, as orchids and the Banyan tree (Ficus Indica), emit 
roots freely from the stem above ground. Cuttings (558) 
and layers (349) form roots because they are maintained in 
contact with abundant moisture and ata suitable tempera- 
ture. Cuttings of some plants, as the willow and nasturtium 
(Tropeslum), root promptly when their stems are immersed 
in water. 
90. Oxygen is Necessary to the Life of Roots. Since 
the cells of newly-formed roots are filled with protoplasm, 
they must have access to the oxygen of the air, or they can 
neither grow nor live. This is shown by a simple experi- 
ment. Boil a quantity of water fifteen minutes, or longer, 
to exhaust it of free oxygen, and then cool it quickly by 
setting it in cold water. Now place a healthy slip of some 
plant that roots freely in water, as willow, nasturtium or 
wandering jew (T7radescantia), in each of two tumblers. 
Pour part of the cool, boiled water into one of tue tumblers 
and add a little olive oil to form a film over the liquid and 
prevent its absorbing more air. Then agitate the rest of 
the water vigorously, to impregnate it again with oxygen, 
and pour some of this into the second tumbler. Set both 
tumblers in a light, warm place. Ina 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. 25.) If now 
