CHAPTER IV. MOVEMENTS OF PLANTS. 25 I 



in part at least to the stimulant effects of light. la the case of a 

 few plants, like the sunflower, when they are young and actively 

 growing, the sensitiveness to light is so great that the leaves and 

 stem follow the sun during his daily course. When, however, we 

 cultivate a plant in such a way that it receives its light chiefly 

 from one direction, as for example when a house-plant is grown 

 in a window, heliotropic movements are very noticeable. The 

 stems, branches and petioles bend over toward the more strongly 

 illuminated side, and the leaf-blades place themselves in a plane 

 at right angles to the rays which fall upon them. If a seedling 

 plant of almost any kind that has an erect habit, be fastened 

 upright in a glass of clear water and placed in a window so that 

 one side is presented toward strong light and the other toward 

 darkness, in a few hours the stem will be bent perceptibly toward 

 the light and the root away from it. The utility of these move- 

 ments in enabling the plant to adjust the position of its organs 

 in such a manner as to make them of the greatest service, is 

 clearly evident. It is not always the case, however, that homo- 

 logous organs behave alike under the same stimulus. The young 

 shoots of the Ivy, when grown in a window, bend away from the 

 light instead of toward it. But here also the movements are of 

 advantage to the plant in enabling it to bring its rootlets into 

 contact with walls, tree-trunks, etc., and so to climb. The nega- 

 tively heliotropic movements of the rootlets of this and other 

 root climbers, and of the tendrils of the Virginia Creeper and a 

 few other climbers, subserve the same end. 



But movements of heliotropism are not confined to multi- 

 cellular organs ; they are often observed in organs composed of 

 a single cell, or even in unicellular plants. The root-hairs of 

 many plants, for example, are negatively heliotropic, while the 

 spore-bearing hyphse of some moulds are positively so, and if 

 certain minute Algae, such as Desmids, which are endowed with 

 the power of locomotion be placed in a glass of water having one 

 side exposed to strong light and the other to comparative 

 darkness, it will be found, after a time, that the Algae have accu- 

 mulated on the illuminated side of the glass. 



Experiment proves that the rays most concerned in producing 

 heliotropic movements are those toward the violet end of the 

 spectrum. 



