DEFENCES OF PLANTS 343 



hairs which are readily detached; a touch with the 

 finger is sufficient to remove a goodly number, and it 

 will be noticed that the detached hairs cling to the skin. 

 A grazing animal that attempts to eat the leaves will 

 have an unpleasant sensation due to the manner in which 

 great numbers of the hairs cling to the mucous mem- 

 brane of its mouth and incommode its activity. Once 

 having had such an unpleasant experience, the animal 

 will in future leave the plants severely alone. In the 

 human mouth the hairs induce almost unbearable irrita- 

 tion. The Comfrey {Symphytum officinale, Plate XXVII.) 

 is armed with stiff bristles which prick like needles, and 

 the beast that can consume the plant in comfort would 

 need to be very hard in the mouth ; other plants belong- 

 ing to the same family — Boraginese — are similarly armed. 

 The stinging hairs of Nettles ( Urtica) are obviously pro- 

 tective; they are ingeniously contrived. Each hair is 

 a tubular, flinty needle terminating in a tiny knob which 

 closes the tube. When touched the knob breaks off, 

 ajid the point of the needle pierces the skin. An irritant 

 fluid is contained in a sac at the base of the needle, 

 and the pressure upon the latter causes the fluid to be 

 forced through the tube and injected into the flesh. 

 The stinging-hair of a nettle is a natural hypodermic 

 syringe. It operates on exactly the same principle as 

 the poison fang of a viper. The British Nettles 

 {Urtica urens and U. dioica) have a sting which is irri- 

 tating, but not severe, yet it is sufficient to ward off 

 browsing animals. Curiously enough, the larvae of the 

 small Tortoiseshell Butterfly {Vanessa urticce) can feed 

 upon these Nettles with impunity, but the damage 

 which they do is neghgible. The stings of Urtica 



