FORMS OF ROOTS. 63 



extremities, which are constantly renewed, so that the minute fibrils 

 serve only a temporary purpose, and represent deciduous leaves. 



121. Roots, in some instances, in place of being subterranean, be- 

 come aerial. Such roots occur in plants called Epiphytes, or air- 

 plants (gV/, upon, and tpvrov, a plant, from growing on other plants), 

 as Orchidaceee; also in the Screw-pine (fig. 115, 2), the Banyan (Fi- 

 cus indica), and many other species of Ficus, where they assist in 

 supporting the stem and branches, and have been called adventitious or 

 abnormal. In Screw-pines, these aerial roots follow a spiral order 

 of development. In Mangrove trees, they often form the entire 

 support of the stem, which has decayed at its lower part. The name 

 of adventitious has also been applied to those roots which are formed 

 where portions of stems and branches, as of the Willow and Poplar, 

 are planted in moist soil. They appear first as cellular projections, 

 into which the fibres of the stem are prolonged, and by some are said 

 to proceed from lenticels (*([ 63). They frequently arise from points 

 where the epidermis has been injured. A Screw-pine in the palm- 

 house of the Edinburgh Botanic Garden, had one of its branches in- 

 jured close to its union with the stem. This branch was at the 

 distance of several feet above the part where the aerial roots were in 

 the course of formation. At the part, however, where the injury had 

 been inflicted, a root soon appeared, which extended rapidly to the 

 earth, and now the branch is firmly supported. 



122. Green-coloured aerial roots are frequently met with in endo- 

 genous 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. In parasites, or plants which derive 

 nourishment from other plants, such as Dodder (Cuscuta), roots are 

 sometimes produced in the form of suckers, which enter into the cellu- 

 lar tissue of the plant preyed upon. 



123. 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. 



124. 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 tap-root is pro- 

 duced (fig. 119). 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 

 ends abruptly, thus constituting the prcemorse (prcemorsus, bitten) root 

 of Scabiosa succisa; or is twisted, as in the contorted root of Bistort. 



