THE PLANT-BODY 



29 



FIG. 21. Aerial roots 

 of Mangrove. (After 

 BAILEY.) 



are the numerous buttress-roots which grow from the base of the 

 trunk in many Palms, and in. the curious Screw-pines (Pandanus). 

 On a small scale the same thing occurs in 

 Indian Corn, and the Mangroves of tropical 

 swamps also offer examples of such aerial 

 roots (Fig. 21). 



Another type of aerial roots is seen in 

 some epiphytic Orchids, whose fleshy roots 

 hang free in the air, from, which they 

 absorb moisture, especially through the 

 spongy tissue of the root-cap. These roots 

 sometimes develop more or less chlorophyll, 

 and then may function also as assimilative 

 organs. Hoot-tendrils, like those of Ivy 

 (Fig. 20, B) and other similar forms, are 

 especially abundant among certain tropical 

 climbers, such as the Araceae. 



Roots of Parasites. Many parasitic plants 

 attach themselves to other plants into which 

 they send their roots, which become more 

 or less modified into suckers, or Haustoria. 

 In Dodder (Cuscuta) these haustoria pene- 

 trate the stem-tissues of the host, while in root-parasites, like Beech- 

 drops (Epiphegus) and Gerardia, the haustoria are connected with 

 the roots of the host. 



Trichomes 



Under the name Trichome are comprised the hairs and similar 

 outgrowths which are developed from the -superficial cells of the 

 plant. The simplest of these are single elongated cells, but they may 

 assume various shapes and sizes. Hairs may be unicellular or multi- 

 cellular, simple or branched, and sometimes are tipped by a gland 

 which secretes mucilage or an essential oil, as in many species of 

 Geranium and Pelargonium (Fig. 22). 



Epidermal scales differ from hairs in having cell-divisions in two 

 planes. Like the hairs, they may be glandular, e.g. the chaffy 

 scales or palese on the young parts of many Ferns. Shield-shaped 

 or peltate scales sometimes occur, and may quite cover the surface of 

 certain leaves ; e.g. Shepherdia (Buffalo-berry) and Elseagnus (Fig. 

 22, E). 



Emergences 



Differing from the trichomes in not being of strictly epidermal 

 origin, are the " emergences," of which the commonest are the 

 prickles and spines on the stems of many plants. The prickles on 



