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. 





Fig. 389. 



Fig. 390. 



Fig. 391 



Fig. 389. — Edge of a leaf of a sedge (Carex strict a), 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, jr, 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 



