118 APPLIED BIOLOGY 



" four-o'clock " and others open in late afternoon; and the 



primrose at sunset. 



P.< 

 1 



Home-work: Keep a list of the plants which you have an op- 

 portunity to observe, and note the hours when the flowers are seen 

 open. ' 



Leaves of many plants (oxalis, clovers, bean, etc.) droop 

 or fold in darkness and assume the so-called " sleep" position. 

 Some so-called " compass plants " avoid the intense noonday 

 sun by moving their leaves so that the edges are vertical and 

 in the north-south direction. 



Another form of external stimulus affecting plants is that 

 of gravitation. That the stems of most plants ordinarily 

 grow upward and the roots downward is a familiar fact. 

 Experiments made by growing young plants attached to ro- 

 tating wheels prove that this direction of growth of plants 

 in a state of nature is due to gravitation. 



Plants also respond to water. Roots will turn away 

 from dry soil and grow in the direction of greater moisture. 



Still other forms of the responses of plants are the numer- 

 ous cases of the twining of stems and the movements of 

 tendrils and special roots in order to aid in climbing. 



Tropisms. We see that in many different ways plants 

 have irritability and respond to external stimuli. The re- 

 sponses in plants are much slower than in animals, but they 

 are none the less definite. These reactions of plants to 

 stimuli are often known as tropisms (from a Greek word 

 meaning to turn). Turning in response to light is helio- 

 tropism (literally, turning to the sun), or phototropism ; 

 to heat is thermotropism ; to gravity is geotropism (literally, 

 turning to the earth) ; to water is hydrotropism ; to chemicals 

 is chemotropism ; to electricity, which seems to have little 

 influence on plants in nature, is electrotropism. The same 

 terms are used in describing the reactions of animals to the 

 various kinds of stimuli. 



