IIG 



BOTANY 



PAllT 1 



sometimes remain on fully developed parts of plants to protect 

 them against too rapid evaporation and direct insolation. Hairs 

 which have this function are usually appressed to the surface of the 

 plant which bears them, while those which serve for flight, climbing, 

 or defence, stand out more or less from the surface (^^^). The hairs 

 developed from some of the epidermal cells of the seed-coats of various 



Fifi. 12o. — Seed-hairs of the cotton, Oossypium her- 

 liaceiim. A, Part of seed-coat with hairs (x 3). 

 Bi insertion and lower part, Bn middle part, and 

 J?a I'Pl"^!' iwti of a hair, (x 300.) 



Fig. 120. — Stinging hair of Urtica 

 dioieu, with a ])ortion of the e]ii- 

 dermis, and, to the right, a small 

 bristle, (x 00.) 



species of Gossi/pium attain an unusual length, and supply the cotton 

 of commerce (Fig. 125). These cotton hairs are sometimes 6 cm. long, 

 and in their fully developed state contain only air ; their cell walls 

 arc thicker than those of ordinary liairs, and covered with a delicate 

 cuticle. They are usually somewhat flattened and at the same time 

 twisted ; and are wider in the middle than at either end. Only a 

 certain number of the epidermal cells of the seed grow out as cotton 

 hairs (Fig. 1 25 7^). JiiiiSTLES are short, pointed hairs, in the thickened 



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