Leaves are sometimes modified to perform other functions than the vital processes: they may be tendrils, as the terminal leaflets of pea and sweet pea; or spines, as in barberry. Not all spines and thorns, however, represent modified leaves: some of them (as of hawthorns, osage orange, honey locust) are branches.
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Fig. 120.—Excluding Light and CO_{2} from Part of a Leaf
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Fig. 121.—The Result.
Suggestions.—To test for chlorophyll. 84. Purchase about a
gill of wood alcohol. Secure a leaf of geranium, clover, or other
plant that has been exposed to sunlight for a few hours, and, after
dipping it for a minute in boiling water, put it in a white cup with
sufficient alcohol to cover. Place the cup in a shallow pan of
hot water on the stove where it is not hot enough for the alcohol
to take fire. After a time the chlorophyll is dissolved by the
alcohol, which has become an intense green. Save this leaf for
the starch experiment (Exercise 85). Without chlorophyll, the
plant cannot appropriate the carbon dioxid of the air. Starch and photosynthesis. 85. Starch is present in the green leaves
which have been exposed to sunlight; but in the dark no starch
can be formed from carbon dioxid. Apply iodine to the leaf from
which the chlorophyll was dissolved in the previous experiment.
Note that the leaf is colored purplish brown throughout. The leaf
contains starch. 86. Secure
a leaf from a plant
which has been in the darkness
for about two days.
Dissolve the chlorophyll as
before, and attempt to stain
this leaf with iodine. No
purplish brown color is produced.
This shows that
the starch manufactured in
the leaf may be entirely
removed during darkness.
87. Secure a plant which
has been kept in darkness
for twenty-four hours or
more. Split a small cork
and pin the two halves on opposite sides of one of the leaves, as
shown in Fig. 120. Place the plant in the sunlight again. After
a morning of bright sunshine dissolve the chlorophyll in this leaf
with alcohol; then stain the leaf with the iodine. Notice that the
leaf is stained deeply except where the cork was; there sunlight and
carbon dioxid were excluded, Fig. 121. There is no starch in the
