PLANT PHYSIOLOGY, 109 



rectly to gravitation (paragraphs 171 to 173). The 

 movement toward the earth is termed geotropism, and 

 organs exhibiting it are said to be geotropic. Organs 

 which move away from the earth, then, exhibit negative 

 geotropism, and are said to be negatively geotropic. 



Practical Studies. Here refer again to tlie experiments on page 99 

 under the topic " Gravitation." 



187. Heliotropism. — In like manner the movements of 

 plants or their parts due to the light are included under 

 the term heliotropism. Organs which turn toward the 

 light are heliotropic (or sometimes positively heliotropic), 

 while those which turn away from it are said to be nega- 

 tively heliotropic, and the phenomenon is negative helio- 

 tropism. The upper surface of most leaves is positively 

 and the lower negatively heliotropic; yet some leaves have 

 both surfaces positively heliotropic, and their blades are 

 therefore approximately vertical and parallel with the 

 meridian, as is notably the case in the compass-plant 

 {Silphium laciniatum) of the prairies of the United States. 

 The tendrils of many plants are negatively heliotropic, as 

 are also the runners of some others. 



188. The movements of plants with the decrease in the 

 amount of light, as at nightfall, often called the "sleep of 

 plants," (nyctitropism) are heliotropic in their nature. 

 Some of these are quite marked, as in many of the clovers, 

 beans, peas, and their allies. The species of Oxalis are 

 notable for these movements. 



189. In regard to the sleep of plants, observation has 

 shown that at night the cotyledons of many plants take a 

 different position from that which they have during the 

 day. In the cabbage and radish, for example, the cotyle- 

 dons stand during the day almost at right angles to the 



