*CHAPTER IV 
- ROOTS ! 
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, 
tropeolum, French marigold, geranium (pelargonium), 
tradescantia, 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 sozl-roots, 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. 
2Tf 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. 
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