THE LEAF IN EELATION TO ITS ENVIRONMENT 305 



meddler with the plant gets stung. Our picture (fig. 361) represents a hair 

 in section. It consists of a long, tapering single cell, rising from a cushion- 

 like base, and widening at the apex into a little knob, which is bent some- 

 what out of the perpendicular. At the point where this bend takes place 

 the cell-wall is extremely thin so thin that a very slight touch suffices to 

 break off the knob. When, therefore, such a touch is given, the mischief 

 is done, and the acrid 

 irritating fluid contained 

 in the cell escapes at the 

 point of rupture and 

 enters the tiny wound 

 which the vitreous apex 

 of the hair has made. 

 The fluid consists of 

 formic acid and a sort 

 of unorganized ferment 

 or enzyme, the latter 

 being thought to be the 

 more poisonous property 

 of the two. It may be 

 added that the break takes 

 place obliquely (a conse- 

 quence of the bend above 

 described), so that the 

 broken end resembles the 

 poison-fang of a serpent 

 to which, indeed, it has 

 sometimes been com- 

 pared. 



A brush from the leaf 

 of any of the British 

 Nettles ( Urtica dioica, 

 tirens, and pilulifera) is 

 doubtless a light matter, 

 but to be stung by some 

 of the Asiatic species is 

 a very different thing. The great Shrubby Nettlfe (Urtica crenulata) 

 of Northern India, for instance, is a nettle to beware of. "This plant," 

 says Sir Joseph Hooker in his Himalayan Journals, " called ' mealum-ma,' 

 attains fifteen feet in height ; it has broad glossy leaves, and though 

 apparently without stings, is held in so great dread that I had difficulty in 

 getting help to cut it down. I gathered many specimens without allowing 

 any part to touch my skin; still, the scentless effluvium was so powerful that 

 mucous matter poured from my eyes and nose all the rest of the afternoon in 

 n 2 



Photo by] 



[E. Step. 



FIG. 372. HAIRY GALLS ON BEECH, 



Caused by a two-winged fly (Hormomyia). 



