EOOTS AND THEIE 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 

 (Elodea), 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 



aerial roots. In some cases, 



as in corn (fig. 22) and in the 



mangrove tree, which grows 



along tropical coasts, the aerial 



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 



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 supply from dangling aerial 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. 



FIG. 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 



