THE HAIKS OF PLANTS. 



C7 



in the latter there is a bulging at the free end (Fig. 112,3), or immediately beloir 



the hard sharp-pointed apex (Fig. 112, e), which co mmuni<;ates with ^ 



the other cells of the hair, or at the base of the hair, and contains a 



poisonous juice. "Whenever such a hair is seized the sharp point 



enters the skin, and the end breaks off immediately below the point, 



and the contained fluid is emitted with a great impetus into the 



wound produced by the puncture. The juice in the perfect hair is 



maintained at a high state of tension, so that it may be emitted with 



violence, after the fashion of the poison in the poison-fangs of the 



serpent. 



It will be inferred, from these remarks, that there must be a circu- 

 lation of the sap in all kinds of hairs. Such is the case ; and the 

 circulation proceeds in currents from the base to the apex of the leaf 

 and back again (Fig. 115, B). It may be seen proceeding under the 

 microscope in the Tradescantia virginica, and appears to proceed 

 between an internal and an external wall of tissue. At a certain 

 period, a cytoblast (page 9) may be detected, and then the current 

 appears to proceed from and return to it. 

 "When the hair has emitted its contents it 

 shrivels, and in some instances (Fig. 116) 

 retracts like the parts of a pocket-telescope. 

 Hairs are not found upon roots, nor upon 

 any part of the plant which is buried in the 

 ground or covered by water ; and whenever 

 they appear on one side of a leaf only, it is, with 

 few exceptions, on the under side. When a 

 portion only of any surface is covered by 

 them, it is uniformly the ribs or veins. They 

 are sometimes found within the cells of water 

 plants, as of the white and yellow water-lilies, 

 Nymphcea alba and Nuphar luteum. Their 

 functions appear to be that of promoting 

 perspiration and of absorbing moisture, inde- 

 pendently of that of secreting fluids. 



Hairy surfaces have received various 



names, according to the nature of the hairs which cover them, as rough, silky, arachnoid 

 (resembing a cobweb), stellate, bearded. The hairs themselves are also variously desig- 

 nated ; thus, stings when they emit an acrid juice, and glandular hairs when the end is 

 tipped with a fluid exudation (Fig. 112 b}. Hooks, barbs, bristles, and velvet are terms 

 which explain themselves. Cilia are long and sparse hairs, arranged in a row on the 

 margin, as in the horse-leek, Sempervivum tectorum. Hairiness expresses a form of hair 

 f a rather long and soft character, as seen in the common hemp nettle (Galeojpsit 

 tetrahii) ; pilosity, when the hairs are longer and more erect, as in the carrot (Daucus 

 carffta) ; and villous, when very long, straight, erect, and soft, as in the JSpilobium. 

 The term tomentum expresses a mass of hairs entangled and closely pressed to 

 the skin, as in the Geranium rotundifol'ium. The longest hairs are probably 

 those which envelop the cotton seed (Gossypium, Fig. 62, B), and constitute 

 the cotton of commerce. They are also very long on seeds of the ccitton tree^, 



Fig. 115. Stinging Hairs. 



A, 1, club-shaped hair, filled with the poison- 

 ous .secretions of the Stinking Hellebore 

 (Helleborus fastidus). 2, a pimilar 

 hair, which has discharged its contents, 

 ai'd then collapsed. 



B, pointed one-celled hair of the WIOANDIA 

 URKNS, filled with poison. The dotted 

 lines show the current of the circulation, 

 and the arrows its direction. 



