NATURE OF PLANTS 41 



able to meet the loss by root absorption. That these plants may 

 have some difficulty in absorbing sufficient water, owing to a 

 lack of proper gas interchange in the roots, is indicated by the 

 curving up into the air of the roots of the bald cypress (Ta.vo- 

 dium) to form the so called knees. These outgrowths are com- 

 posed of loose spongy tissues and probably supply the roots with 

 air. At least they are. not formed when the trees grow in suffi- 

 ciently dry and aerated soil. For the same reason the roots of seed- 

 lings grown in too damp soil grow up to the surface of the soil. 

 It has also been suggested that these aquatic xerophytes at one 

 time lived under really arid conditions and owing to causes not 

 known they have become restricted to bogs and marshes with- 

 out loss of xerophytic characters. 



