36 BIOLOGY 



very different reactions in different kinds of proto- 

 plasm. 



Some of the slime fungi in the early stages of their 

 life always avoid the light and seek out the shade, 

 while later on in life they seek out the light, illustrating 

 the fact " that the same stimulus may induce different 

 reactions in the same protoplasm at different stages in 

 its life." 



We may lay down a third general rule, which can be 

 illustrated equally well in plants and animals, namely, 

 that the varying intensities of the same stimulus pro- 

 duce different effects in the same protoplasm. 



We have seen that the response to stimuli in animals 

 is more rapid and more marked. This is due to the 

 presence, in addition to a general sensitivity, of special 

 sense organs; that is, tissues or parts of tissues that 

 have become differentiated both in structure and in 

 function for the sole purpose of receiving special classes 

 of stimuli. Thus we have the eye for the appreciation 

 of light and shade, the ear for sound, and special organs 

 for taste, touch, and smell. 



Why is it that the plant world is practically devoid of 

 such organs ? Self-preservation, and through that the 

 preservation of the species, is of vital importance to 

 every living organism. It must obtain the necessary 

 quantity of suitable food ; it must satisfy all its needs 

 with regard to air and water ; and it must have a suit- 

 able amount of heat if it is to live its life in a healthy 

 manner. The difference is due to the different mode of 

 life ; the animal is active while the plant is sedentary ; 

 the food of the plant is abundant everywhere, while 

 the animal can live only on organic food which is, 



