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. — Placc 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. 2 31. — a toward or away from light." It includes 

 plant that has been ^^ j^jj^^jg ^f jj j^^ adjustments, and examples 



growing near an open ^ . . , 



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

 lowa'^rrthe^Hght"''"''^ disposition of Icavcs 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. 



