66 ECOLOGY Part II 



3. Metabolism. Even in higher plants the rate of metabolism is low. In 

 active respiration the temperature of plants may rise only slightly above their 

 environment. In higher animals the rate of metabolism is high. The temper- 

 ature of birds and mammals is usually much higher than that of their environ- 

 ment. 



In the majority of animals, there are special organs of excretion by which 

 nitrogenous waste products of metabolism are eliminated. In plants, there are 

 no such organs. The only approach to an excretory product in plants is prob- 

 ably the excess by-products of metabolism such as opium (see similarities of 

 plants and animals). There are no excretory organs in plants. 



4. Hormones. Plants produce relatively few hormones and these have 

 general effects, such as, growth of stem and growth of root. Animals produce 

 an elaborate and delicately adjusted series of interacting hormones which have 

 specific effects, such as, thickness of skin. 



5. Responsiveness. In plants, the ordinary cells are variously responsive, 

 e.g., to light, to temperature, in some regions more than others. In animals, 

 special sensory cells are highly responsive to one or another kind of stimulus, 

 e.g., the rod cells and cone cells of the eye. 



