CHAPTER IV. 
Roots.! 
38. Origin of Roots. —The primary root originates from 
the lower end of the caulicle, as the student learned from his 
own observations on sprouting seeds. The branches of the 
primary root are called secondary roots, and those which occur 
on the stem or in other unusual places are known as adventi- 
tious roots. The roots which form so readily on cuttings of 
willow, southernwood, Tropeolum, French marigold, geranium 
(pelargonium), and many other plants, when placed in damp 
earth or water, are adventitious. 
39. Experiment 13.— Place in water cuttings of any kind of 
plant which roots readily, and sketch at intervals of two or three days 
the roots which are formed. 
40. Aerial Roots.— Those roots which are formed in the 
air are called aerial roots. They serve various purposes, — 
in some tropical air-plants, Fig. 13, they are known to absorb 
moisture and other useful substances from the air and 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 sup- 
ports ;* many such air-plants are shown in the frontispiece. 
In such plants as the ivy, Fig. 14, the aerial roots (which 
are also adventitious) hold the plant to the wall or other 
surface up which it climbs. 
1To the plant the root is more important than the stem. The author has, how- 
ever, 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 knowledge of the 
more complex anatomy of the stem will serve every purpose. 
2 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. 
