PLANTS AND THE LIGHT 21 
Observation and study.—Selecting the milkweed, sun- 
flower, or branch of horse-chestnut, note and sketch the 
form and size of the leaves. If variation in form or size 
occurs, show this in the notes and sketches. Does the 
petiole (the leafstalk) vary in length? Are any leaves sessile 
(without petioles)? Is there any definite arrangement of 
leaves upon the stem—in pairs, fours, alternate, or opposite? 
With the top of the plant directed toward the eye, see 
whether all the leaves are placed so that they would get 
sunlight at one time. Observe from a side view. Are all 
leaf-blades parallel to the surface of the earth? Are they 
the same on young and old parts of the stem? Compare 
the sunflower and the ragweed in these respects. What 
advantage is gained by the position in each case? Are the 
leaves which you have examined arranged for receiving the 
rays of light that strike the plant parallel with the stem, 
or at right angles to it? 
With these points in mind, make comparisons with the 
other plants mentioned as to the form, size, and position of 
leaves and their consequent lighting. Of what advantage 
are partially divided leaves, as in those of the oaks, maples, 
and ragweeds? Is this a common device among plants? 
LESSON VI 
Leaves and the light—trees 
Materials. Such common trees as maple, elm, chestnut, 
cottonwood, oak, pine, spruce, etc. 
Observation and study.—Examining a vigorous maple 
or oak, note the general outline of the stem and branches. 
Are branches mainly upright or horizontal? In this re- 
spect, does the tip of the branch differ from the rest of the 
axis? Are the leaves arranged around the branch, as in 
the milkweed and the sunflower? Is this arrangement the 
