SUB-AiiRlAL ROOTS 129 



rise to the various forms of sub-aerial roots, or those that 

 grow above ground. 



183. Water Roots. — Large numbers of plants are 

 adapted to live in the water, either floating freely, as the 

 duckweed {Lemna) and bladderwort {Utriciilaria), or an- 

 chored to mud and sticks on the bottom. Water roots are 

 generally white and threadlike and more tender and suc- 

 culent than ordinary soil roots. Many land plants will 

 develop water roots and thrive on that element if brought 

 into contact with it. Place a cutting of wandering Jew in 

 a clear glass of water, and in from four to six days it will 

 develop beautiful water roots in which both hairs and cap 

 are clearly visible to the naked eye. 



The chief office of ordinary roots being to absorb mois- 

 ture, they have a great affinity for water, and its presence 

 or absence exerts a strong determining influence on their 

 direction, often overcoming that of geotropism (Sec. 169). 



184. Parasitic Plants are those that live by attaching 

 themselves to some other living organism, from which 



they draw their nourishment 

 ready made. Their roots are 

 adapted to penetrating the 

 substance of the host, as their 



267 268 



267, 268. — Misiletoe penetr,iting bough of oak : 267, lower part of stem attached 

 to branch ; 268, longitudinal section through one of the bauistoria strands, showing 

 its progress as the branch thickens. 



victim is called, and absorbing the sap from it. They are 

 appropriately named haiistoria, a word meaning suckers, 

 or absorbers. Dodder and mistletoe are the best-known 

 examples of plant parasites, though the latter is only 

 partially parasitic, as it merely takes up the crude sap, 

 from the host and manufactures it into food by means of 

 its own green leaves. 



Andrews's bot. — 9 



