STRUCTURE.] STRUCTURE OF HAIRS. 157 



also be mentioned as exhibiting a similar instance. When a 

 portion only of the surface of any thing is covered by hairs, 

 that portion is uniformly the ribs or veins. According to 

 De Candolle, hairs are not found either upon true roots, 

 except at the moment of germination, nor upon any portion 

 of the stem that is formed under ground, nor upon any parts 

 that grow under water. 



In a very large number of hairs, perhaps in all, there may 

 be seen, at some period of their existence in any cell, a cyto- 

 blast, and a circulating system, formed of numerous fine 

 streams, which all appear to proceed from and return to the 

 cytoblast itself. (See Plate II. fig. 13. 14. 18.) In the monili- 

 form disarticulating hairs of Polystachya, already described, 

 each joint of the hair has this structure in a very remarkable 

 manner. 



If hairs are examined with low magnifying powers, their 

 sides appear to be simple, and they are accordingly regarded 

 as mere expansions or attenuations of the vesicles of the 

 epidermis : but if they are studied with more attention and a 

 more powerful microscope, it becomes evident that their sides 

 are double ; for currents may often be seen streaming along 

 their sides, and evidently interposed between the external 

 smooth surface and an uneven interior membrane. This is 

 easily observed in the jointed hairs of Tradescantia virginica, 

 where the nature of the current is distinctly shown by 

 minute molecules, that are carried along by the stream. If 

 the hair of Tradescantia is suffered to die on the field of the 

 microscope, and dry up, it then becomes evident that it is 

 composed of two sacs, the one firm and external, the other 

 extremely thin, and after death contracting so much as to 

 leave a considerable space between its sides and the external 

 sac. (See Plate II. fig. 14. b.) It appears to me that this 

 is the general structure of all hairs in which a circulation of 

 sap, and the cytoblast are both visible ; and it is probable 

 that the external sac is the cuticular membrane, hard, firm, 

 and scarcely capable of shrivelling ; while the internal sac is 

 protoplasm, thin-sided, and not acquiring any firmness with 

 age, but shrivelling up as soon as the fluid which distends it 

 when alive is withdrawn. 



