66 THE ROOT. 



from the joints of creeping or prostrate plants ; as the ground- 

 ivy, and the twin-flower (Linnsea). 2d, The roots of certain 

 erect plants of the endogenous structure, originating from the 

 stem liigh in air, descending and entering the soil. Of tliis class 

 the screw-pine (Pandanus) is a remarkable example, whose 

 aerial roots are often several feet in length before reaching the 

 earth. Such roots, a few inches in length, are also seen in the 

 common maize (Zea). 



b. A third class of aerial roots is peculiar to the epiphytes {s^t, 

 upon, (fVToy, a plant). These plants are fixed upon the trunk 

 and branches of other species, and derive their noiurishment 

 cliiefly from the air. Such are the long moss (Tillandsia), pen- 

 dent from lofty trees, and many of the Orcliidacese at the south. 

 4th, The roots of parasites are usually aerial. These are not 

 only attached to other vegetables, but, penetrating their tissues, 

 they derive nourishment from their juices. The Cuscuta and 

 Mistletoe are examples. 



PHYSICAL STRUCTURE AND FUNCTIONS OF THE ROOT. 



151. The internal structure of the root is similar to that of the 

 stem (q. v.), except that there is often a greater proportion of 

 cellular, fleshy matter, as in the beet. In Endogens the root 

 is endogenous, in Exogens it is exogenous, but in the latter case 

 it is always destitute of a pith. 



152. The fibrils are in fact but subdivisions of the caudex, or 

 main root. They consist of minute bundles of vasiform tissue 

 (32), enclosed in a loose, cellular epidermis, except at the ex- 

 tremities (35), where the tissue is naked and becomes exceed- 

 ingly loose and spongy. These (spongioles) have the property 

 of powerfully absorbing water. 



153. The growth of the root does not take place by the ex- 

 pansion of the parts already formed, but simply by the addition 

 of new matter at the extremities, and by the formation of new 

 layers upon the surface. Tliis accounts for the facility with 

 which it penetrates the crevices of the soil, and forces its way 

 into the hardest earth. 



154. The most obvious function of the root is the purely 

 mechanical one of fixing the plant in the earth, and maintaining 



