178 PRACTICAL COURSE IN BOTANY 



keep them in the dark for several days ; the leaflets will fall away, leaving 

 a clear scar like those on winter twigs. 



EXPERIMENT 76. To SHOW THAT ADJUSTMENTS TO TEMPERATURE MAY 

 BE MADE BY CHEMICAL MEANS. Place a small twig of oleander, laures- 

 tinus, or other broad-leaved evergreen in a 5 to 10 per cent solution 

 of sugar, and transfer it at the end of a few days to a temperature of 

 6 to 8 below freezing. On comparison with a similar twig that has 

 stood for the same length of time in pure water, it will be found to possess 

 a greater power of resistance to cold. 



194. The light relation. The principal external con- 

 ditions to which leaves have to adjust themselves are light, 

 air, moisture, gravity, temperature, and the attacks of ani- 

 mals. From the knowledge of their work and function 

 gained in the preceding sections, it will be clear that the pri- 

 mary relation of the leaf is a light relation, and to this, first of 

 all, it must adjust itself. 



It was shown in Exps. 56 and 57 how promptly leaves re- 

 spond to changes in the direction of light, 

 and a little observation (Exp. 74) will con- 

 vince us that they are equally sensitive to 

 changes in intensity and periodicity of illu- 

 mination. 



195. Phototropism. - - The movement of 

 plants in response to light is called photo- 

 tropism a word that means " turning 

 FIG. 231. A toward or away from light. " It includes 



plant that has been jj fo^fa f ft^t adjustments, and examples 



growing near an open t . . 



window, showing the of it are to be met with everywhere in the 



toward ^he^Hght! 11 l disposition of leaves with reference to their 



light exposure. 



196. Horizontal and vertical adjustment. - - Take two 

 sprigs, one upright, the other horizontal, from any convenient 

 shrub or tree and notice the difference in the position of 

 the leaves. Examine their points of attachment and see how 

 this is brought about, whether by a twist of the petiole or of 

 the base of the leaf blades, or by a half twist of the stem 

 between two consecutive leaves, or by some other means. 



