CHAPTER IV 

 ROOTS 1 



39. 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 adven- 

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

 willow, southernwood, tropaeolum, French marigold, gera- 

 nium (pelargonium), tradescantia, and many other plants, 

 when placed in damp earth or water, are adventitious. 



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

 plants grow in the earth and are known as soil-roots, there 

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

 They serve various purposes : in some tropical air-plants 

 often cultivated they fasten the plant to the tree on which 

 it establislies itself, and tliey also 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. 



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

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

 roots of orchids and aroids may be examined. 



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