356 



BRITISH FLOWERING PLANTS 



2 inner ones larger than the outer. There are three 

 British species. U. dioica, the Common Nettle, is 

 perennial ; the other two annual ; one U. urens, with 

 the flowers in nearly sessile short clusters ; the other, 



U. piluUfera, with the 

 male flowers in loose 

 spikes, the female in 

 stalked globular heads. 



U. urens and U. pilu- 

 lifera are monoecious, 

 with stinging hairs. 



U. dioica (Common 

 Stinging-nettle) is gener- 

 ally dioecious. The plant 

 is protected by stinging 

 hairs (Fig. 230). These 

 point forwards, so that 

 the plant may safely be 

 grasped from below, as 

 they are merely com- 

 pressed. If, however, 

 taken from above, they 

 are sure to run into the 

 skin. Each hair sits on 

 a cushion of delicate 

 tissue, contains an acid 

 fluid, and terminates in 

 a small rounded head set 

 on at an angle. The 

 silicified tissue at the 

 base of the head is very 

 thin, and breaks through 

 with the slightest touch. 

 As the fracture is oblique (Fig. 231, a b) the point is very 

 sharp, so that it easily penetrates into the flesh. Our 

 British Nettles can make themselves very unpleasant, 

 but some of the foreign species produce serious results. 

 Borscow ^ has described the movements of the proto- 



' Bull. Acad. Sci. St. Petersburg, xii. (1868). 



'■f-^v'yx 



Pig. 230. rig. 281. Fig. 232. 



Fig. 230. — Urtica dioica. Vertical section 

 through part of the leaf of a Stinging- 

 nettle, showing two hairs, from the lower 

 one of ■which the head has been broken. 

 X 35. 

 Fig. 231. — Top of hair more magnified, entire. 



X 150. a b, line of fracture. 

 Fig. 232. — Top of hair more magnified, after 

 removal of the head, x 150. 



