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. 
Exprerimment 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 alight relation, and ts 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 
Fria. 231.—A toward or away from light.” It includes 
plant that has been J] kinds of light adjustments, and examples 
growing near an open 
window, showing the of it are to be met with everywhere in the 
oye nclisht "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. 
