CHAPTER IV 
ROOTS? 
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, tropeolum, French marigold, gera- 
nium (pelargonium), tradescantia, and many other plants, 
when placed in damp earth or water, are adventitious. 
40. Aérial 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 aérial roots. 
They serve various purposes: in some tropical air-plants 
often cultivated they fasten the plant to the tree on which 
it establishes itself, and they 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 Tf it can be conveniently managed, the class will find it highly interesting 
and profitable to visit any greenhouse of considerable size in which the aérial 
roots of orchids and aroids may be examined. 
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