130 



DERMAL SYSTEM 



a 



genus Urtica the stiff and brittle condition is mainly due to silicifica- 



tion, so far as the distal part of the hair is concerned ; the proximal 



portion, on the other hand, is calcified. In the Loasaceae the walls 



of the stinging-hairs are calcified, while in the genus Jatropha they are 



strongly lignified. In all cases the structure of the tip displays several 



interesting adaptive features ; this subject has received 



special attention at the hands of the author. Typically a 



stinging-hair ends in a minute swollen head ; this terminal 



swelling breaks off at the slightest touch, leaving an open 



point, which can readily enter the body of the aggressor 



(Fig. 38 a,b). The head is more or less spherical or oval, 



and is attached obliquely; in the Loasaceae it is usually 



so small as to be hardly distinguishable from the rest of 



the distal region (Fig. 30 g). On looking at the tip of a 



stinging-hair from the side, one is at once struck by the 



fact that its walls are of very unequal thickness in different 



parts. In the case of Urtica dioica a narrow strip of the 



convex side, situated just above the slight constriction or 



neck, is remarkably thin and often very sharply separated 



from the neighbouring thickened portions of the wall 



(Fig. 38 a). On the concave side of the tip the wall is 



also relatively thin in one place ; but in this case the 



fig. 37. difference is not so marked, and, moreover, the thin area 



stinging-hair of is much more extensive and passes over gradually into 



the thicker portion. The tip of the stinging-hair of Loasa 



papaverifolia is constructed on a very similar plan ; here the extensive 



unthickened region of the concave side is extraordinarilv thin 



(Fig. 38 o). 



An important feature of the mechanism of stinging-hairs depends 

 upon the fact that the tip normally breaks away along a plane which 

 includes the above-mentioned thin areas : in other words, the precise 

 line of separation is predetermined by the structure of the wall. This 

 arrangement not only facilitates the detachment of the tip, but also 

 ensures that the resulting point has the shape which is best suited for 

 the purpose of piercing the object that it encounters. Owing to the 

 fact that fracture takes place along an oblique plane, the resulting 

 point is lancet-shaped and very sharp, and the aperture from which 

 the poison issues is situated laterally some distance behind the point 

 (Fig. 38 b). The open point of a stinging-hair is thus seen to be 

 constructed after the pattern of the cannula of a hypodermic syringe 

 or like the grooved poison-fang of a snake. In Loasa papaverifolia 

 and in Jatropha stimulata the portion of the cell- wall which forms the 

 actual sharp point after the tip is broken off is considerably thicker 



