68 CELLULAR TISSUE. 



Sect. 13. The cells of Hair-structtires are, while young, provided like other 

 young cells with a well-developed protoplasmic body, and many while in this condition 

 quickly attain a great size, so that they are specially suitable and easily obtained 

 objects for the study of the protoplasmic body. Mature hairs behave in two different 

 ways as regards their protoplasmic body and their contents. Those of the first 

 category resemble, in short, the epidermal cells, having a permanent protoplasmic 

 body, usually in the form of a delicate sac-like lining to the cell-wall ; more rarely 

 the protoplasm persists for a longer time in considerable quantity (stinging hairs of 

 Urtica, Hairs of Cucurbita, &c.). The cavities in the protoplasm are permanently 

 filled with watery cell-sap (Sap-containing Hairs). In hairs of the second category 

 the protoplasm and cell-sap dry up when growth stops, and are replaced by air. 

 These persist as air -containing hair-structures. The capitate hairs containing 

 mucilage in Osmunda regalis are hitherto unique, and will be described below 

 (Sect. 19). 



All root-hairs and a large number of the hair-structures which occur on foliage 

 leaves contain cell-sap. They can be distinguished at once from those of the other 

 category by their transparency. Their protoplasmic body and contents show the 

 same series of various modifications of special character as is the case in the epidermal 

 cells. Most of them, e.g. all root hairs, all (?) stinging hairs, &c., are devoid of 

 chlorophyll. Others have more or less abundant grains of chlorophyll and allied pig- 

 ments. The correspondence with the epidermal cells extends also, as far as is known, 

 to the substances mixed with the cell-sap (comp. Weiss, /. c. 645). 



The contents of the often-described stinging hairs have special peculiarities, 

 which are also said to occur in many hair-structures described as glandular hairs of 

 various categories. 



We know of the erect stinging hairs of the Urticaceae, Loasese, and other plants 

 named above (page 60), which resemble one another so remarkably in structure and 

 form, that the brittle point (Sect. 22) breaks off when touched, and that a fluid issues 

 through the hole thus made, which causes more or less slight inflammation when 

 applied to the human skin, especially if it enters the small wounds caused by touching 

 the hair itself. It is further known of this fluid that it has, like most cell-fluids, an 

 acid reaction, not alkaline as stated formerly ^- On the fact that by distilling the 

 nettle plant with sulphuric acid formic acid is obtained, the conjecture has been 

 founded that the latter substance causes the phenomena of stinging ^. But as a matter 

 of fact nothing is known of the active substance, not even whether it is to be sought 

 for in the acid fluid, or in the protoplasm '. 



The apical cells of capitate hairs are often distinguished by very dense proto- 

 plasmic contents, in which resinous substances may be shown to be present. Hanstein 

 (Bot. Ztg. /. c. p. 748) states that in the multicellular capitate hairs of Salvia all the 

 cells may finally be united by the solution of their membranes into one fluid mass 

 (containing Resin or Balaam) which is surrounded by the bladder-like cuticle. In the 



' P. de CandoUe, Physiologie, libers, v. Roper, I. p. 193. 



* Von Gorup-Besanez, in Journ. f. pract. Chemie, XL VIII. p. 191. 



* Compare the neat paper of Duval-Jouve (which however gives no new information), 'Sur les 

 stimules d'ortie,' in the Bulletin Soc. Bot. France, XIV, p. 36. 



