126 ORGANOGRAPHY. 



oi'dinary roots contained in the soil, but after having arrived 

 at a certain age these perish, and they then derive it entirely 

 from the plants upon which they grow ; others again continue 

 throughout their life to derive a portion of their nourishment 

 by means of roots imbedded in the soil. 



We have now described the general characters and structure 

 of the true or primary root, and the secondary or adventitious root. 

 We have in the next place to allude to certain differences which 

 roots present which are dependent upon their duration. Roots 

 are thus divided into annual, biennial, and perennial. 



1. Annual Roots. — These are produced by plants which grow 

 from seed, flower, and die the same year in which they are 

 developed. In such plants the roots are always of small size, 

 and either all spring from a common point, as in annual Grasses 

 (Jig. 240); or the true root is small, and gives off from its sides 

 a number of small branches. Such plants, in the process of 

 flowering and matm-ing their fruits and seeds, exhaust all the 

 nutriment they contain, and thus necessarily perish. 



2. Biennial Roots. — These are produced by plants which 

 spring from seed one year, but which do not flower and ripen 

 their seeds till the second year, when they perish. Such roots 

 are commonly enlarged in various ways at the close of the first 

 season (^fig. 250), in consequence of their tissue becoming gorged 

 with nutritious matters stored up for the support of the plant 

 during its flowering and fruiting the succeeding season. The 

 Can-ot {Jig. 248) and Turnip {fig. 250) afford us good examples 

 of biennial roots. 



3. Perennial Roots. — These are the roots of plants which live 

 for many years. In some such plants, as the Dahlia {fig. 244), 

 Orchis {fig. 242), &c., the roots are the only portions of the plant 

 which arc tlms perennial, their stems, &c., dying down to the 

 ground yearly. Such perennial roots are either of woody con- 

 sistence, or more or less fleshy as in biennial roots. 



We have seen in treating of the stem that that organ possesses 

 certain differences in its internal structure in the three great 

 classes of Dicotyledonous, Monocotyledonous, and Acotylcdon- 

 ous Plants. The roots of such plants in like manner possess 

 similar distinctive characters, and also some otlicrs, which render 

 it necessary for us briefly to refer to them. 



1. The Root of Dicotyledonous Plants. — The roots of 

 these ))hit)ts are formed, as we have seen, by the direct elonga- 

 tion of tlie radicle of the embryo (p. 119). Such a mode of root- 

 development has been called by Richard and other botanists 

 exorhizal. 



It follows from this mode of development that Dicotyledonous 

 Plants liave generally a ta})-root or descending central axis {fig. 

 233) from wliich branches arc given off in various directions, in 

 the same manner as such i)lauts have also an ascending axis or 



