120 OEGANOGRAPHT. 



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

 roots present depending upon their duration. Roots are thus 

 divided into annual, biennial, and jpenwiial. 



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 

 {fig. 233) ; 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 maturing their fruits and seeds, exhaust all the 

 nutriment they contain, and thus necessarily perish. 



2. Biennial Roots. — These are produced by plants M'hich 

 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, in consequence of their tissues becoming gorged with 

 nutritious matters stored up for the support of the plant during 

 its flowering and fruiting the succeeding season. The Carrot 

 {fig. 241) and Turnip {fig. 243) 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. 237), 

 Orchis {fig. 235), the roots are the only portions of the plant 

 which are tlius perennial, their stems dying down to the ground 

 yearly. Such perennial roots are either of woody consistence, 

 or more or less fleshy as in those of biennial plants. In the case 

 of fleshy roots such as the Dahlia and Orchis, the individual roots 

 are not in themselves perennial, but usually perish annually ; but 

 before doing so, they produce other roots from some point or 

 points of their substance, hence the whole root is perennial, 

 although any particular portion may perish. Woody roots are 

 commonly perennial in themselves, and are not renewed. 



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

 certain differences in its internal stmcture in the three great 

 classes of Dicotyledonous, Monocotyledonous, and Acotyledonous 

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

 distinctive structural characters, and also some others, which, 

 although generally referred to previously, had better be briefly 

 summed up here. 



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

 these plants are formed, as we have seen, by the direct elonga- 

 tion of the radicle of the embryo. Such a mode of root-develop- 

 ment has been called exorkizal, and a root thus formed is called 

 a inie root. 



It follows from this mode of development that Dicotyledonous 

 Plants have generally a tap-root or descending axis {fig. 227) 

 from which branches are given off in various directions, in 

 the same manner as such plants have also an ascending axis or 



