78 ON THE PLANT-CELL. 



of the epidermal processes are the stinging hairs. They constitute 

 the type of a very common form of the epidermal tissue, in which a 

 few wart-like cells are elevated above the surface, and embrace the base 

 of a single elongated cell (figs. 92, 93. a). 



Such hairs, very much thickened and distinguished by the porosity 

 of the epidermal cells, are seen in Dipsacus (fig. 93.); ordinarily 

 the lower cells of such hairs are swollen, with thin walls, whilst those 

 above are pointed and thick-walled. They are frequently marked on 

 the upper surface with little warts spirally arranged, and with elevated 

 stripes. This form characterises the Urticacece, the Boraginacece, the 

 Cucurbitacece, and the Loasacece. The mechanism also of the stinging 

 hairs in Urtica, Wigandia urens, and the Loasacece, is very interesting. 

 Almost all stinging hairs end in a little knob-shaped swelling, which 

 is exceedingly brittle, and easily knocked off by a touch. The opened 

 point, on being pressed against, exudes the secretions contained in the 

 cells at the base of the hair, and will produce poisonous effects when 

 introduced into animal tissues. Our indigenous nettles are the least 

 injurious. The stings of the Loasacece are much more so, while the Urtica 

 crenata and crenulata of the East Indies produce wounds in which 

 pain is felt for weeks and months after touching them. The most dan- 

 gerous of all is the Urtica urentissima of Blume, called in Timor Daoun 

 setan, and by the English " Devil's leaf." The wounds of this plant give 

 pain for years after, especially in damp weather, and occasionally death 

 from tetanus is the result. Could we separate this poison, it would be 

 the most powerful vegetable poison known. 



In the early stages of growth these hairs, all of them, possess an active 

 circulation of the sap. Some hairs have their contents absorbed at a 

 special time, so that the hair is, as it were, absorbed into its own proper 

 cavity. This remarkable phenomenon 

 takes place in the hairs of the style in 

 Campanulacece* (fig. 94.). Also in the 

 globular cells of knob-shaped hairs, which 

 then look as if half had been cut through, 

 or as if a cover had been removed.f 

 Meyen has published a work on hairs, 

 distinguished by a host of peculiarities.^ 



Cork. A peculiar change goes on in 

 the epidermal cells of particular parts, 



more especially the stem and fruits of Jf&l x^nJJ 15""") 



certain trees. A quantity of yellow slimy 

 matter collects in the cells and gradually 

 increases in quantity, so that the external 

 cell-wall is torn by the under one and lifted 

 above the surface. Cells are formed in 

 a hitherto undiscovered manner in the 



yellow substance, which assume the form of four- cornered tables, and are 

 arranged in connected concentric layers. When perfectly formed, this 



* See Brongniart, Ann. de Sc. Nat., 1839, p. 244. 



f According to Meyen; but it is erroneous. 



j Ueber die Secretionsorgane der Pflanzen. Berlin, 1837. 



94 Longitudinal section through the style of a Campanula, with two hairs : a, a hair 

 exhibiting a circulation ; its point is enclosed in a layer of mucus : b has lost its con- 

 tents, and is in consequence contracted. 



