CHAPTER IV 

 ROOTS 1 



48. Origin of Roots. — The primary root originates from 

 the lower end of the hypocotyl, as the student learned 

 from his own observations on sprouting seeds. The 

 branches of the primary root are called secondary roots, 

 and the branches of these are known as tertiary roots. 

 Those roots which occur on the stem or in other unusual 

 places are known as adventitious roots. The roots which 

 form so readily on cuttings of willow, southernwood, 

 tro;^olum, French marigold, geranium (pelargonium), 

 tradescaiI1»a»,and many other plants, when placed in damp 

 earth or water, are adventitious. 



49. Aerial Roots. —While the roots of most familiar 

 plants grow in the earth and^are known as soilrroots, there 

 are others which are formed in the air, called aerial roots. 

 They serve various purposes : in some tropical air-plants 

 (Fig. 13) they serve to fasten the plant to the tree on 

 which it establishes itself, as well as 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 supports ; ^ many such 



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

 however, 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 knowl- 

 edge of the more complex anatomy of the stem will serve every purpose. 



i* 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. 



36 



