132 DERMAL SYSTEM 



certain tropical Nettles (such as Urticu stimulans in JaVa and U. 

 urentksima in Timor) suffice to disprove the old theory. According 

 to the author, the poison of Urtica dioica is an albuminoid substance, 

 held in solution in the cell-sap, and in many respects resembling 

 an enzyme, especially as regards its solubility. 



Concerning the mode of liberation of the poisonous cell-contents it 

 may be remarked that the elastic tension of the cell-wall of the bulb 

 which is neither calcified nor silicified is sufficient to cause active 

 ejection. Duval-Jouve long ago observed that, when the tip of a 

 stinging- hair of the Nettle is broken off by a touch with a needle, a 

 little drop of cell-sap at once exudes, or is even forcibly ejected. 

 There can, however, be no doubt that the pressure exerted upon the 

 bulb by the impinging object helps to produce ejection of the cell-sap. 

 Among the reasons for this conclusion is the circumstance that one 

 may be perceptibly stung twice in succession by the same hair ; 

 turgor obviously cannot have any share in inflicting the second 

 sting. 



In Urtica dioica the cells forming the basal cup are remarkably 

 well provided with chloroplasts ; the cup should therefore probably be 

 regarded as a local photosynthetic apparatus pertaining to the hair. 

 This view is supported by the fact that numerous pits are present in 

 the thickened lateral walls of the bulb, a circumstance which points to 

 the existence of an active interchange of material between the cup- 

 cells and the principal cell, or body, of the hair (Fig. 38 c). 



In many plants it is only the young leaves that possess a protective 

 coat of hairs, the adult organs being entirely devoid of such a covering : 

 this statement applies more especially to the upper surfaces of foliage 

 leaves (Tussilago Farfara, Petasitcs nivciis, etc.). Whether the hairs are 

 cast in such cases, merely because they become superfluous when the 

 cuticle and cutinised layers are fully developed and when the chloro- 

 phyll apparatus no longer requires protection, or because the persistence 

 of a hairy covering would deprive the leaf of too large a proportion of 

 the available light, or would otherwise interfere with its functions, is 

 a question which must be left undecided. Detailed researches carried 

 out by Keller 75 upon the phenomena attendant upon normal depilation 

 have shown that this process follows one of two different methods. 

 In the case of unicellular trichomes and sometimes also in the 

 multicellular types the hair breaks off immediately above its point of 

 insertion ; the base of the hair is therefore laid open. In such cases 

 the persistent basal walls usually become thickened and cutinised 

 before separation takes place : in certain cases they may even acquire 

 all the characteristics of typical outer epidermal walls. According to 

 Keller special arrangements which facilitate the breaking off of a hair, 



