THE LEAP 13t 
more carbohydrate than the plant can deal with, so 
that, as we have seen, this is temporarily stored up in 
the form of starch. At night, it is on this temporary 
store that the plant draws, not only to build its 
substance, but to supply, by oxidation, the energy 
required for its vital processes. Therefore, during 
darkness, the starch is re-converted into sugar, so that, 
after the light has been excluded from the plant for 
some time, starch will be found to have disappeared 
from the leaves. 
Definite experiments relating to the formation and 
storage of starch may easily be made. 
Certain minerals appear to be necessary. We 
found by water eultures that plants grown without 
potash were more or less stunted, and further investi- 
gation shows that this is chiefly owing to the fact 
that without potash photosynthesis does not take place. 
Then again, though iron does not enter into the 
composition of chlorophyll, yet chlorophyll is not 
formed in its absenee, and since, without chlorophyll, 
there is no photosynthesis, iron is essential to this 
process. 
The absence of chlorophyll is obvious to the eye, 
but the fact that starch is not being formed must be 
proved in a way that will be indicated in connection 
with the next experiment. 
To show that light is needed for photosynthesis, 
partly cover with some opaque substance leaves that 
are growing in the bright sunlight. To do this, obtain 
a small flat stand, (a block of wood will do), lay on 
this a piece of soft loose felt that will allow air to 
pass freely between its fibres, and then, without 
detaching the leaf from the plant, lay its blade on 
the felt, and cover part of it as directed. A good 
plan is to place a stencil on the leaf, so that the light 
reaches the surface only where the pattern is cut 
(Fig. 82). The leaf should then be left till the 
