ROOTS AND RHI20IDS 



Soi 



to cause geotropic curvature (see p. 464, for a discussion of the statolith theory of 

 starch; also figs. 697, 698). 



The duration of roots. — The classification of plants into annuals, 

 biennials, and perennials is based chiefly upon the length of hfe of roots, 

 though many herbs with perennial underground stems have annual roots. 

 The conditions that determine duration are not known, though a little 

 light is thrown on the subject by the behavior of annuals and biennials. 

 A biennial is a plant that lives in two vegetative periods, while a.n- annual 

 lives in but one. Many plants are 

 annuals or biennials, depending in 

 part upon the time of germination ; 

 for example, winter wheat is a 

 biennial, and spring wheat an an- 

 nual. Other plants, as the pepper- 

 grass and the shepherd's purse, ex- 

 hibit similar phenomena. Probably 

 most hardy annuals become bien- 

 nials if planted in late summer, 

 while many biennials become an- 

 nuals if planted in early spring, 

 and many more if started yet 

 earlier in a hothouse. When an- 

 nuals and biennials are grown in 

 uniform conditions, as in the moist 

 tropics or in a moist greenhouse, 

 they become perennials in many 

 instances, while certain plants (as 

 the castor bean) that commonly 

 are perennials in uniform climates 

 tend to become annuals in periodic climates. Some annuals (as Poa 

 annua and Senecio vulgaris) become biennials or perennials when 

 transferred to alpine habitats. In some trees, as the silver poplar, 

 osage orange, and redwood, new shoots arise one after another from 

 the old roots, so that the tree lives long after the first trunk, has 

 rotted away. Indeed, so far as root duration is concerned, some 

 perennials may be said to possess a capacity for perpetual life. 



The influence of external factors upon root form and develop- 

 ment. — Transplanting in relation to root form. — Reference has been 

 made to the changed direction assumed by lateral roots when the tap 



Fig. 712. — A portion of the creeping 

 stem of a water pennywort {Hydrocotyle) 

 showing adventitious roots grouped at the 

 nodes, each leaf with its node and group 

 of roots representing a potential plant; the 

 leaves have long petioles, which react 

 readily to changes in the direction of the 

 incident light. 



