332 



A TEXT-BOOK OF BOTANY. 



primary roots develop. The biennials are nearly all dicotyledons 

 and have a persistent primary root which, while usually slender, 

 may become fleshy, as in Beta. In the perennials, on the other 

 hand, we find a number of different types of roots, varying from 

 the slender aerial roots of epiphytes to the smaller tuberous, 

 fleshy roots of many terrestrial plants, and the peculiar roots of 

 parasites. 



Alterations in the Form of Plants. — The shoot, in its 



Fig. iqo. Specimens of "orris root" of commerce consisting of peeled pieces of the 



rhizomes of Iris florenlina. The rhizomes are mostly dichotomous, the branches becoming 

 obconical and of characteristic shape. The large circular areas terminating the rhizomes 

 are scars of stem bases, while the small black dots on the surface are scars from attachment 

 of roots. 



course of development, is subject to a great many hostile external 

 conditions, and there results more or less mutilation and alteration 

 in the form of the plant. One of the most destructive influences 

 to plants is that of strong winds when they attain the velocity 

 of gales. They stunt the growth of woody plants and cause the 

 branches to assume a horizontal position (Fig. 191). This un- 

 usual growth is further accentuated if at times during the winter 

 they are covered with snow and sleet. If they grow in close 



