STEUCTaEE OF EOOTS. 



39 



and hairs are often seen in roots, but no true leaves. These hairs 

 consist of simple elongated cells, which occur singly, and appear to serve 

 the purpose of absorption (fig. 97, h h h). Eoots increase principally 

 by additions to their extremities, which are constantly renewed, so 

 that the minute fibrils serve only a temporary purpose, and represent 

 deciduous leaves. The tissue at the extremities of roots is older and 

 more dense than that immediately below it, so as to form a protecting 

 covering. 



Eoots, in some instances, in place of being subterranean, become 

 aerial. Such roots occur in plants called Epiphytes, or air-plants (i-jrl, 

 upon, and (pvrhv, a plant, from growing on other plants), as in Orchi- 

 dacese ; also in the Screw-pine (Pandanus), (fig. 98), the Banyan 

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

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



Kg. 98. 



Fig. 99. 



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

 of development. In Mangrove trees (fig. 99) they often form the 

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

 name of adventitious is applied to roots arising from the sides of 

 stems, as for instance those which are formed when portions of stems 

 and branches of the Willow and Poplar are planted in moist soU. 

 They appear first as cellular projections, into which the fibres of the 

 stem are prolonged, and by some are said to proceed from lenticels. 

 They frequently arise from points where the epidermis has been in- 

 jured. A Screw-pine, in the paJm-house of the Edinburgh Botanic 

 Garden, had one of its branches injured close to its union with the 



" Fig, 98. Pandanus odoratissimus, the Screw-pine, giving off numerous aerial roots near 

 the tase of its stem. Fig. 99. Ehizophora Mangle, the Mangrove tree, supported, as it 

 were, upon piles, by its numerous roots, which raise up the stem. The plant grows at the 

 muddy mouths of rivers in warm climates. 



