160 APPLIED BIOLOGY 



as some orchids and ferns common in greenhouses, are at- 

 tached to branches of trees, but do not have parasitic roots 

 penetrating and absorbing sap from the supporting tree. 

 Instead, their roots absorb water from the moist air and also 

 that which drips from leaves and branches. 



(2) Prop-Roots. On some plants roots start from the 

 stem above ground and grow down into the soil so as to 

 form bracing or supporting structures. Every one who has 

 cultivated Indian corn must have noticed the brace-roots 

 or prop-roots which start from nodes above the soil and grow 

 down into the soil. These help the ordinary roots in bracing 

 the tall stalk (stem) against winds, and also it is probable 

 that they advantageously increase the number of rootlets 

 available for absorption of water while the plant is maturing. 

 Other examples of roots forming supports or braces are in 

 the famous banyan trees of India, whose aerial roots descend 

 from branches to the ground and form many additional sup- 

 ports and absorbing roots for the tree. A single tree may 

 cover a space 300 feet in diameter. Mangrove trees, which 

 grow along the sea-coast in tropical regions, form brace- 

 roots from both main stem and branches, and these roots 

 grow down into the mud. 



(3) Water Roots. Roots of many plants cannot live in 

 water. This is the reason why many trees die when a new 

 dam has raised the water level. Some plants, however, have 

 roots which can live in either soil or water, e.g., willows. 

 Many aquatic plants have roots adapted to living in water 

 only, e.g., duckweed and water-hyacinth. 



Many aquatic roots have no root-hairs. Such roots are 

 usually much branched and have the required amount of ab- 

 sorptive surface without root-hairs. Some aquatic plants 

 can live entirely without roots, absorbing by their sub- 

 merged stems and leaves. 



(4) Breathing Roots. Reference has already been made 

 ( 105) to the fact that roots get some oxygen from air in 



