RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 751 



that the root proceeding from the embryo perishes as soon as it has emerged 

 from the seed, and that then many new roots originate from the hypocotyl, 

 <jlose to the place from which sprang the dead primary root; or that roots are 

 developed on the lower end of the epicotyl embedded in the ground — in which 

 case they stand closely crowded together in great numbers forming a cluster, and 

 are then known botanically as fascicled roots {radix fasciculata). But many roots 

 also arise further up on the shoot-axis, not only in the region of the scale-leaves, 

 but also, if required, in the foliage portion of procumbent, erect, and climbing 

 stems, and under certain conditions even on the foliage-leaves. These structures 

 which may originate from the stem at all stages of age and height, and even from 

 the leaves, are called adventitious roots {radices adventicice). 



When roots are developed on a leafy stem, it is noticed that their places of 

 origin are near the points of insertion of the leaves. In epiphytes, especially in 

 aroids and orchids living on the bark of trees, they are sometimes seen to be so 

 distributed that a single root, a pair of roots, or a whole fascicle of roots, arises at 

 exactly defined places on the stem. Each intemode in these plants has its own 

 roots, and is therefore almost independent of neighbouring internodes, so that, 

 supposing one or both the adjoining internodes should die from some cause or 

 other, it can maintain itself independently {cf. fig. 51, p. 224). In stems which 

 lie on the ground, as in runners, the roots always originate only at the nodes, 

 i.e. at the commencement of an internode. In the underground stems known as 

 rhizomes, the roots are distributed in the same way. When the older internodes 

 of these runners and rhizomes die off behind, the next youngest are not injured, 

 for they are already provided with roots of their own, by whose help their 

 requirements of water and food-salts are supplied, and by which they are firmly 

 fixed in the gi-ound. The general symmetry and geometrical distribution of the 

 places of origin, as shown in leaves, is, however, absent in the majority of roots, 

 the arrangement being frequently quite irregular, especially in underground, much- 

 branched roots where influences operate on them which will be spoken of later. 



The functions assigned to the root are: first, the absorption and transport of 

 water and of food-salts dissolved in water, and second, the fixing of the whole 

 plant in the substratum. In most cases this twofold function is performed by 

 the same root, but occasionally a division of labour occurs, so that one portion 

 of the root-system serves only for the absorption of food, and another for the 

 fixing in the substratum. For instance, the repeatedly mentioned Tecoma radicals 

 has two kinds of roots; the first underground, absorbing water and food-salts from 

 the soil, and the second the clinging roots (figured on p. 479), by which the light- 

 avoiding shoots are attached to places from which no fluid nourishment could 

 possibly be absorbed. When one of these shoots is cut across below the place 

 at which it is fixed by roots to a wall or rock-face, the part above the section 

 forthwith dries up, even although these roots and the substratum are kept con- 

 tinually moistened and damp. 



Roots of biennial and perennial plants, in regions where the vegetative activity 



