KOOTS 
95 
tern, as if seeking for water and the essential materials. 
Sometimes the root system becomes remarkably extensive, 
visiting a large amount of soil in order to procure the 
necessary supplies. Sometimes the soil is poor in heat, and 
root activity is interfered with. In such cases it is very 
common to find the leaves 
massed against the soil, thus 
slightly checking the loss of 
heat. 
Most soil roots also need free 
air, and when water covers the 
soil the supply is cut off. In 
many cases there is some way 
by which a supply of free air 
may be brought down into the 
roots from the parts above 
water ; sometimes by large air 
passages in leaves and stems 
(see Figs. 87, 88, 89, 90) ; some- 
times by developing special root 
structures which rise above the 
water level, as prominently 
shown by the cypress in the 
development of knees. These 
knees are outgrowths from roots 
beneath the water of the cypress 
swamp, and rise above the water level, thus reaching the 
air and aerating the root system (see Fig. 91). It has been 
shown that if the water rises so high as to flood the knees 
for any length of time the trees will die, but it does not 
follow that this is the chief reason for their development. 
65. Water roots. A very different type of root is devel- 
oped if it is exposed to free water, without any soil relation. 
If a stem is floating, clusters of whitish thread-like roots 
usually put out from it and dangle in the water. If the water 
level sinks so as to bring the tips of these roots to the mucky 
FIG. 90. Longitudinal section 
through a young quillwort leaf, 
showing that the four air cham- 
bers shown in Fig. 89 are not con- 
tinuous passages, but that there 
are four vertical rows of promi- 
nent chambers. The plates of 
cells separating the chambers in 
a vertical row very soon become 
dead and full of air. In addition 
to the work of aeration these air 
chambers are very serviceable in 
enabling the leaves to float when 
they break off and carry the com- 
paratively heavy spore cases. 
