122 BOTANY 
following day and picked about the middle of the 
afternoon. It should now be placed in boiling water 
for a minute or two, for the purpose not only of 
expelling the air it contains, but of killing its proto- 
plasm, so that there may be no further changes in its 
organic compounds. ‘Thus, if starch is present, we 
shall be sure of finding it even if the leaf is left for 
a considerable time. Now place the leaf in methylated 
spirit and leave it for some time. If the leaf is a soft 
one, like those of the so-called nasturtium, it will bleach 
in the course of an hour or so, but if it be boiled in the 
spirit it will bleach at once. We now have a perfectly 
colourless leaf in a solution of chlorophyll. On removal 
from the spirit the leaf will be found to be very brittle. 
Lay it on a white saucer and wash it with water till it 
again becomes soft, and then pour iodine on it. The 
parts that were exposed to the sun will shortly show a 
blue black colour, indicating the presence of starch. 
Thus the pattern of the stencil is marked out on the 
leaf, the part that was covered remaining unaffected by 
the iodine. Light is therefore necessary for starch 
formation. 
Another method is to place flat slices of cork 
opposite each other on either side of a growing leaf 
and fix them to the leaf by passing a couple of pins 
right through both corks and leaf (Fig. 83). While 
starch will be found in the exposed parts of the leaf, 
the portion covered by the cork will contain none. 
This is not so satisfactory as the previous experiment, 
since the corks pressed against the leaf surface 
hinder the access of air. 
Chlorophyll is essential to photosynthesis, for it 
is this substance alone that can absorb and utilise the 
energy of the sunlight. This is easily shown. In the 
bright sunlight pick variegated geranium leaves 
(Fig. 84) and treat them as in the last experiment, 
noting that they take much longer to bleach than the 
