CHAPTEE IV. 



Roots.i 



38. Origin of Boots. — The primary root originates from 

 the lower end of tlie caulicle, as the student learned from his 

 own observations on sprouting seeds. The branches of the 

 primary root are called secondary roots, and those which occur 

 on the stem or in other unusual places are known as adventi- 

 tious roots. The roots which form so readily on cuttings of 

 willow, southernwood, Tropsolum, French marigold, geranium 

 (pelargonium), and many other plants, when placed in damp 

 earth or water, are adventitious. 



39. Experiment i>3. — Place in water cuttings of any kind of 

 plant which roots readily, and sketch at intervals of two or three days 

 the roots which are formed. 



40. Aerial Boots. — Those roots which are formed in the 

 air are called aerial roots. They serve various purposes, — 

 in some tropical air-plants, Pig. 13, they are known to absorb 

 moisture and other useful substances from the air and to 

 take in water which drips from branches and trunks above 

 them, so that these plants require no soil and grow in mid- 

 air suspended from trees, which serve them merely as sup- 

 ports ; ^ many such air-plants are shown in the frontispiece. 

 In such plants as the ivy. Fig. 14, the aerial roots (which 

 are also adventitious) hold the plant to the wall or other 

 surface up which it climbs. 



1 To the plant tlie root is more important than the stem. The author has, how- 

 ever, treated the structure of the latter more fully than that of the root, mainly 

 because the tissues are more varied in the stem and a moderate knowledge of the 

 more complex anatomy of the stem will serve every purpose. 



2 If it can be conveniently managed, the class will find it highly interesting and 

 profitable to visit any greenhouse of considerable size, in which the aerial roots of 

 orchids and aroids may be examined. 



