122 Introduction to Botany. 



morning, at noon, and in the late afternoon of a single 

 day. It can be seen, by comparing the same leaves 

 in the different photographs, that they were incessantly 

 striving to intercept as much light as possible by keeping 

 their broad surfaces at right angles to the changing direc- 

 tion of greatest illumination. Such movements, and any 

 others caused by light, are designated as heliotropic, and 

 the state or condition of plants which makes heliotropic 

 movements possible is termed heliotropism. The student 

 may observe many other plants which exhibit high degrees 

 of motility of this kind. 



95. Influence of Various Forces. The movements ex- 

 hibited by plant members, for which light and gravity are 

 the guiding forces, are of the most vital significance to the 

 plant; but there are movements which are directed by 

 other means. Observation 99 has taught us that roots 

 may disregard gravity and turn in the direction of moist 

 areas; and we saw by Observation 103 that by contact 

 with an object, tendrils of some plants may be made to 

 grow on one side more than on another so that they twine 

 about the object if it is of suitable size and shape. There 

 are other movements due to unequal growth, exhibited 

 chiefly by the parts of flowers, which are induced by vary- 

 ing degrees of light and heat. The flowers of Tulip, Crocus, 

 Colchicum, etc., will open in a few minutes if the tempera- 

 ture is raised to a marked degree, and at a constant tem- 

 perature they will open in the light and close in the dark. 

 It is a matter of common observation that the dandelion 

 behaves in this way. 



96. Motor Organs. Old plant members which exhibit 

 movements are usually provided with special structures for 

 that purpose. A description of the behavior of the leaves 

 of scarlet runner, and of their motor organs, will serve to 



