66 THE ROOT. 



firom 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 high in air, descending and entering the soil. Of this class 

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

 aerial roots are often several feet in length before reacliing 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 pecuhar to the epiphytes (ent, 

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

 and branches of other species, and derive their nourishment 

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

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

 4th, The roots o£ 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. Tlie 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 gi-owth 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 fonnation of new 

 layers upon the surface. This accounts for the facility with 

 which it penetrates the crevices of the sod, 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 



