CHAPTER IV 



ROOTS 



26. Origin of roots. The primary root originates from the 

 lower end of the hypocotyl, as the student learned from h^s own 

 observations on sprout- 

 ing 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 ad- 

 ventitious roots. The 

 roots which form so 

 readily on cuttings of 

 willow, southernwood, 

 Tropseolum, French marigold, 

 cultivated " geranium " (Pelargo- 

 nium), Tradescantia, and many 

 other plants, when placed in 

 damp earth or water, are adven- 

 titious. 



27. Aerial roots. While the 

 roots of most familiar plants 



grow in the earth, there are others which are 

 formed in the air, called aerial roots. They serve various pur- 

 poses : in some tropical air plants (Fig. 13) they fasten the plant 

 to the tree on. which it establishes itself, as well as take in 



19 



FIG. 13. 



Aerial roots of 



an orchid 



