ANIMALS AS FOOD, FOES, OH FRIENDS. 267 



milky juice of plants like milkweeds, which often contains 



acrid substances, may also be protective. 



379. 3. Mimicry. — Certain plants ' 



which are not distasteful or disagreeable 



have adopted the same form of leaves 



and stem and the general habit of those 



which grazing animals have found dis- 

 tasteful. This mimicry causes them 



to be avoided, as vs^ell as the really 



hurtful ones which they imitate. 



380. 4'. Ants.— In the 

 tropics particularly, cer- 

 tain, plants secure them- 



FlG. 227. Fig. 228. Fig. 229. 



Fig. 227. — Edge of a leaf of a sedge (,Carex stricia), showing alternate epidermal cells 

 pointed and underlaid by two layers of mechanical cells. Magnified 200 diam. — After 

 Kemer. 



Fig. 228. — Part of a shoot of barberry in spring showing leaves of preceding year as 

 persistent three-pointed thorns, in whose axils the buds are developing into the sea- 

 son's shoots. Najtural size. — After Kemer. 



Fig. 229. — A stinging hair of the nettle {Urticn), in longitudinal section, x, emerg- 

 ence in which the singlecceiled hair al/t- is sunk below n6. The knoblike apex c is 

 easily broken off because the cell wall just below it is thin and brittle. The oblique 

 cutting edge left pierces the skin like a hypodermic needle and some of the acrid cell 

 contents enters the wound, causing intense itching. Magnified 60 diam.— After 

 Frank. 



selves from the attacks both of browsing animals and leaf- 

 cutting insects by encouraging the presence of colonies 

 of warlike ants upon them, and making provision for 



