SPECIALIZED ROOTS 
253 
their function, but in 
their position. They can 
grow from any side of 
a stem, enabling them 
to attach themselves to 
a surface wherever they 
may happen to touch it. 
The haustoria by which 
parasite plants (see page 
369) get their food from 
the host are modified 
aerial roots. They are All tap roots are used for food storage, 
able to take food from and most of them are either biennial or 
the stem of the host by P erennial - 
absorbing pads, as is well illustrated by dodder. 
The cypress tree, which grows with its roots submerged, 
has peculiar projections which extend above the surface of 
the water. These so-called “ knees ” are so modified that 
the roots are enabled to 
secure the air they need. 
The pendant roots of the 
tropical orchids, known as 
velamens, have a modified 
outside layer which en-' 
ables them to absorb al¬ 
most immediately any 
water which falls on them 
and conduct it to the cen¬ 
tral cylinder for storage. 
This enables the plants 
possessing them to live 
outside of the soil (see 
page 373, epiphytes). 
Prop roots of corn grow 
from nodes above the soil, 
Figure 243. — Extent of Root System 
of Corn 90 Days After Planting. 
Note how the roots of adjoining hills 
mingle. This shows how keen the 
struggle is to get water and food, and 
how the soil is deprived of its food 
contents. 
Figure 242. —Tap Roots of Radish. 
