13. THE ROOT. 45 



monly radial. In some cases, however, the root is isobilateral, 

 as is shown as well by its internal structure as by the develop- 

 ment of two opposite longitudinal rows of lateral roots. In 

 other cases (e.g. attached aerial roots of epiphytic Orchids ; roots 

 of Podostemacese) its symmetry is more or less distinctly dorsi- 

 ventral. 



Roots branch either dichotomously (e.g. Isoetes), or laterally 

 (see p. 9). In lateral branching the secondary roots are devel- 

 oped in acropetal succession on the primary root, and so on. Since 

 the lateral roots are not developed upon the growing-point of the 

 parent root, the terminal apical portion of the parent root con- 

 sequently bears no lateral roots. On anatomical grounds (see 

 Part II.) the secondary roots are arranged in longitudinal rows 

 on the primary roots ; an ar- 

 rangement which also obtains in 

 the branches of the secondary roots, 

 and of higher orders. 



The form of the root is usually 

 cylindrical ; when it is very delicate, 

 as in Grasses, it said to be fibrous. 

 The primary or the secondary roots 

 may become much swollen, serving 

 as- depositories for nutritive sub- 

 stances ; the Turnip, the Carrot, the 

 Beet, the Radish, have swollen 

 primary roots ; the Dahlia has 

 swollen secondary roots. 



FIG. 30. The lateral roots n aris- 

 ing endogenoualy from the peri- 

 cycle of the tap-root of Vicia Faba 

 (longitudinal sec. mag. 6 times) : 

 / axial cylinder (stele) ; r cortex 

 of the main root; h root-cap. of the 

 lateral root. 



Various terms are employed to de- 

 signate the different forms of swollen 

 roots; that of the Turnip is termed 



napiform ; that of the Carrot, conical ; that of the .Radish, fusiform or 

 spindle-shaped; those of the Dahlia and of some terrestrial Orchids, 

 tuberous. 



Many plants have aerial roots which are peculiar both morpho- 

 logically and physiologically. The roots of epiphytes, that is, 

 plants (mostly Orchids and Bromeliaceae) which grow on trees 

 without, however, being parasitic, never reach the ground, but 

 serve as means of attachment : they frequently contain chlorophyll 

 and serve as organs of assimilation, especially in Podostemacese. 

 Some plants climb by means of aerial roots (e.g. Ivy, Tecoma radi- 



