348 



PLANT LIFE. 



472. 2. Distasteful or injurious substances, such as 

 volatile oils, camphors, acids, tannins, alkaloids, etc. The 

 milky juice of plants like milkweeds, which c 



often contain acrid substances, may also be 

 protective. 



473. 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 graz- 

 ing animals have found distasteful. This 



mimicry causes them to be 

 avoided, as well as the really 

 hurtful ones which they imitate. 



a 



FIG. 389. 



FIG. 390. 



FIG. 391. 



FIG. 389. Edge of a leaf of a sedge (Career strzcta), showing alternate epidermal cells 

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

 Kerner. 



FIG. 390. 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. Natural size. After Kerner. 



FIG. 391. A stinging hair of the nettle (Urtica), in longitudinal section. _r, emerg- 

 ence in which the single-celled hair abc is sunk below ab. 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. 



474. 4. Ants. In the tropics particularly, certain plants 



