36 STUDY OF COMMON PLANTS. 



gravitation, grow down obliquely, while roots of a higher 



order extend indifferently in various directions. The 



result is such a distribution of the root system 



Primary and ' . . . 



secondary as to bring it into contact with the soil far more 

 roots, perfectly than if the roots grew down together 



in a common bundle. It has been noticed, however, that if 

 the end of the primary root is destroyed one or more of the 

 secondary roots near it grow vertically downward to take 

 its place. 1 



While the branches arising from the first or primary 

 root are properly called secondary, the same term is also 

 Adventitious frequently applied to roots of a higher order, 

 roots, an d i s sometimes rather loosely extended to 



those given off by the stem and other parts of the plant. 

 The latter, however, are commonly spoken of as adventi- 

 tious. Aerial roots, such as those of the ivy and trumpet- 

 creeper, properly fall under this head. Other adventitious 

 roots are of great importance in the practical operations 

 of florists and gardeners, enabling them to increase their 

 stock by taking advantage of the capacity of slips and 

 cuttings for promptly forming roots. The readiness with 

 which cuttings of willows and poplars produce adventitious 

 roots, together with their rapid growth, has led to their 

 extensive planting in the western states, and many trouble- 

 some weeds owe their pertinacious hold on the soil to the 

 same habit. 



In a comparatively small number of plants, of which the 

 dodder is a familiar example, adventitious roots take the 

 Parasitic form of suckers which penetrate the tissues of 

 habits, other plants, on which they live as parasites. 



The plant thus attacked is called the host, from the rela- 

 tion in which it stands to its parasite. But few flowering 



1 Darwin, Power of Movement in Plants, p. 196. 



