40 FORMS OF EOOTS. 



stem. This branch, was at the distance of several feet above the part 

 where the aerial roots were in the coui'se of formation. At the part, 

 however, where the injury had been inflicted, a root soon appeared, 

 which extended rapidly to the earth, and then divided so as to form 

 rootlets; thus the branch was firmly supported. The extremities 

 of the aerial roots of Orchids are covered with a layer of delicate 

 whitish tissue, composed of spiral cells. This layer is called vdamen 

 radicum, or covering of the roots. 



Green-coloured aerial roots are frequently met with in endogenous 

 plants. Such roots possess stomata. In the Ivy, root-like processes 

 are produced from the stem, by means of which it attaches itself to 

 trees, rocks, and walls. Those processes are subservient to the pur- 

 poses of support rather than nutrition. In parasites, or plants which 

 derive nourishment from other plants, such as Dodder (Guscuta), roots 

 are sometimes produced in the form of suckers, which enter into the 

 cellular tissue of the plant preyed upon. 



When roots have been exposed to the air for some time, they 

 occasionally assume the functions of stems, losing their fibrils, and 

 developing abnormal buds. Duhamel proved this experimentally, 

 by causing the branches of a willow to take root while attached to the 

 stem, and ultimately raising the natural roots into the air. 



Forms of Roots, 



The forms of roots depend upon the mode in which the axis 

 descends and branches. When the central axis goes deep into the 

 ground in a tapering manner, without dividing, a taproot is produced 

 (fig. 96). This kind of root is sometimes shortened, and becomes 

 succulent, forming the conical root of carrot, or the fusiform, or spindle- 

 shaped root of radish, or the napiform root of turnip, or it is kvisted, 

 as in the contorted root of Bistort. 



When the descending axis is very short, and at once divides into 

 thin, nearly equal fibrils, the root is called fibrous, as in many grasses ; 

 when the fibrUs become short and succulent the root is fasciculated, 

 as in Ranunculus Ficaria and Asphodelus luteus (fig. 100) ; when the 

 succulent fibrils are of uniform size, and arranged like coral, the root 

 is coralline, as in OoraUorhiza innata ; when some of the fibrils are 

 developed in the form of tubercules, containing starchy matter, the 

 root is tubercular ; the tubercules, in such cases, are in reality stem- 

 tubers, as seen in the Jerusalem Artichoke (Helianthus tuberosus), and 

 in Orchis (fig. 101) ; when the fibrils enlarge in certam parts only, the 

 root is nodulose, as in Spiraea Filipendula (fig. 102), or moniliform, as in 

 Pelargonium triste (fig. 103), or annulated, as in Ipecacuan (fig. 104). 

 Some of these so-called roots are formed of a stem and root combined, 



