42 



BOTANY 



as of all roots, is endogenous. Dormant root rudiments occur in the 

 same manner as dormant buds of shoots. The ease with which willows 

 are propagated from shoots is well known, and is due to the prompt- 

 ness with which they develop adventitious roots from apparently 



latent embryonic tissue, when 

 the requisite conditions of 

 moisture and darkness are ful- 

 filled. 



The Metamorphosis of the 

 Root. — The customary nomen- 

 clature for the various root 

 forms is based on their shape, 

 size, and mode of branching. 

 A root which is a prolongation 

 downwards of the main stem 

 is called the main root or TAP- 

 ROOT ; the other roots are 

 termed, with reference to the 



tap-root, LATERAL ROOTS of 



different orders, according to 

 the order of their develop- 

 ment. The roots may enlarge 

 The and become turnip-shaped or 

 tuberous (Fig. 44). Such 

 tuberous growths often greatly resemble stem tubers, but may be dis- 

 tinguished from them by their root-caps, by the absence of any indica- 

 tions of leaf development, and by their internal structure. 



Fig. 44. — Root-tubers of Dahlia variabilis. 

 lower portions of cut stems, (i nat. size.) 



The tubers of the Orchidaceae exhibit, morphologically, a peculiar mode of for- 

 mation. They are, to a great extent, made up of fleshy, swollen roots, fused to- 

 gether and terminating above in a shoot- bud. At their lower extremity the 

 tubers are either simple or palmately segmented. In the adjoining figure (Fig. 

 45) both an old (£') and a young tuber (£") are represented still united together. 

 The older tuber has produced its flowering shoot (6), and has begun to shrivel and 

 dry up ; a bud, formed at the base of the shoot, in the axil of a scale leaf (s), has 

 already developed the adventitious roots, which, swollen and fused together, have 

 given rise to the younger tuber. 



The aerial roots of tropical Epiphytes differ considerably in their 

 structure from underground roots. The aerial roots of the Orchidaceae 

 and of many Aroideae are provided with a spongy sheath, the VELAMEN 

 by means of which they are enabled to absorb moisture from the 

 atmosphere. Aerial roots, in some cases, grow straight downwards, and 

 upon reaching the ground, branch and function as nutritive roots for 

 the absorption of nourishment ; in other instances, they turn from the 

 light, and, remaining comparatively short and unbranched, fasten them- 

 selves as climbing ROOTS to any support with which they come in 



