The Root and the Soil. 67 
are filled with water, because the soil tends to prevent the 
water within its cavities from absorbing air. 
97. Potted Plants should have Abundant Provision 
for Drainage, and the outside of the pots should be kept 
clean, to admit air through their walls. Potting soil should 
contain sufficient sand and humus (93) so that it does not 
readily become puddled by watering. 
98, Potted Plants should be Watered with Care. 
They should receive sufficient water so that the soil particles 
are constantly surrounded with a film of water, but not so 
much that the soil cavities remain filled. 
99. How the Root-Tip Penetrates the Soil. Darwin 
made the interesting discovery that the root-tip, in advanc- 
ing through the soil, does not move in a straight line, but 
has an oscillating motion, which enables it to take advan- 
tage of openings between the soil particles. The force with 
which the root-tip is pushed forward, by increase in length 
of the root, was calculated by Darwin to be at least a 
quarter of a pound, in some cases, while the increase of the 
root in diameter may exert a much greater force. The 
root-tip is protected in its passage through the soil by a 
thimble-like covering called the root-cap.* 
100. Growth of Roots in Length. Since the soil 
offers more or less resistance to the growth of roots, in land 
plants, it is evident that the roots could not elongate 
throughout their whole length at once. On the contrary, 
the part that increases in length is limited to a short por- 
tion just behind the root-tip. Sachs found that the part of 
the rootlet of the broad-bean, that increased in length by 
* The root-cap is readily seen without a magnifying glass when a bean plant is 
grown iu water. 
