STINGING-HAIRS 



131 



than the rest of the distal region (Fig. 38 c, h). In Jatropha stimulata 

 the thin place is characteristically developed on the concave side of 

 the tip, but is altogether absent on the convex face; on this side 

 fracture always occurs at the constriction just behind the tip, where 

 the thickening layers of the wall are sharply bent. It is interesting, 



Fit:. 3S. 



Structure of various stinging-hairs. A-D. Urlica dioica. A. Apex of hair with tip 

 attached. B. Apex after the tip has been broken off. C. Bulb of the hair [and the 

 cup-shaped emergence in which the bulb is embedded] in L.S. D. Base of the haii - 

 cell after treatment with sulphuric acid ; the silicified region of the wall has not 

 swollen. E-H. Apical portions of various stinging-hairs. E. Urtica pilulifera. 

 F. Laportea giijas. G. Loasa papaverifolia. H. Jatropha stimulata. a-b (in A and H), 

 lino of separation of the tip. 



from the phylogenetic point of view, to note that a series of transi- 

 tions may be observed, within the limits of a single family, between 

 an imperfectly constructed tip and the highly specialised structure 

 which has just been described. 



The author has shown that the substance which is injected from 

 stinging-hairs into the wounds that they cause, is not, as was at one 

 time generally assumed, formic acid. As a matter of fact, the very 

 grave toxic effects (namely violent and persistent pain, tetanus-like 

 symptoms and, in extreme cases, death) produced by the sting of 



