ROOTS AND THEIR RELATION TO PLANTS 31 
produce only water roots (if they have roots at all), is rather 
small. Some of the commonest are the so-called water hya- 
cinth and the little duckweeds (fig. 21). 
Plants like the pondweeds, water crowfoot, water weed 
(Llodea), and others, which grow entirely submerged, do not 
need an extensive root system, as they are in no danger of 
drying and so use their roots mainly for anchorage. 
29. Air roots. Many plants which root in the earth, such 
aS corn, poison ivy, and English ivy, produce roots from 
portions of the stem above- 
ground. These are called 
aérial roots. In some cases, 
as in corn (fig. 22) and in the 
mangrove tree, which grows 
along tropical coasts, the aérial 
roots finally reach the earth 
and serve as braces to prop 
the stem of the plant upright. 
In other instances the roots 
never reach the ground, and 
then they may serve to enable 
the plant to climb, as in the 
case of the poison ivy and the 
Fic. 23. Base of stem and sucking 
roots of the mistletoe, growing on an 
apple branch 
In the figure the bark of the branch has 
been removed so as to show how the 
mistletoe roots spread between the bark 
and the wood of the host plant. In the 
section of the branch at the left is shown 
the way in which short portions of the 
mistletoe root penetrate into the wood 
of the host. One half natural size. After 
Bonnier and Sablon 
English ivy, or they may serve 
to anchor the plant to stones or to the bark of trees and at 
the same time to absorb rain water or dew. Many tropical 
air plants are perched on the bark of trees, or even on their 
leaves, and get their water sunply from dangling aérial roots 
which are covered with a layer of absorbent bark that catches 
water and then gradually gives it to the plant. 
30. Parasitic roots. Certain plants, such as the dodders 
(fig. 34) and many kinds of mistletoe (figs. 23 and 35), live 
wholly or partly at the expense of other plants, into which 
their sucking roots, or haustoria, penetrate, sometimes very 
deeply. The mode of life of such parasites will be further 
discussed in Chapter IV. 
