The Structures and Processes of Stems 131 



type of stem grows upright even in dcirkness. If these stems 

 are laid horizontally, the younger parts will grow faster on 

 the lower side and the stem will again become erect. This 

 response of a plant organ to gravity is called geotropism} 

 If the response is in the direction of the pull — that is, toward 

 the earth — as in the case of a primary root, the organ is 

 said to be positively geotropic. If the response is in the op- 

 posite direction, as in most stems, the organ is said to be 

 negatively geotropic. If the response is side wise, as in many 

 branches, the organ is said to be transversely geotropic. 



Stems in relation to light. Light affects also the direction 

 of growth of stems, as the plant grown at a window will show. 

 Most stems grow toward the strongest light. Response to 

 Light is called phototropism, and most stems are positively 

 phototropic. The stems of some prostrate plants are held 

 close to the ground by their response to light, as is proved by 

 the fact that if a shade is placed over them, the stems become 

 erect. Some of the common doorweeds of paths and waste 

 places are examples. There are some prostrate stems, of 

 course, that lie flat on the ground because of a lack of me- 

 chanical tissue to hold them upright. 



The direction of growth in branches is a compromise be- 

 tween the response to light and the response to gravity. In 

 some trees, like the spruce, the direction of growth in the 

 branches is in some way controlled by the main stem. If the 

 top of the main stem is cut off, one or two of the lateral branches 

 become negatively geotropic instead of transversely geotropic. 



^ The responses of organs to external influences like gravity and light are 

 called tropisms (Greek : /ro/>r, turning). "Geotropism" means literally a 

 turning toward or away from the earth; "phototropism," a turning toward 

 or away from light; "hydrotropism," a turning toward or away from water. 



