altering the Daily Period of Light. 91 
is concentrated on what takes place in a single cell of the spongy parenchyma 
of the leaf during daylight. 
At the points where such a cell is in contact with neighbouring cells, 
water and certain inorganic substances in solution, which have been absorbed 
by the roots and have travelled up the vessels of the stem, are passing into 
the cell in question. 
Where the cell-wall abuts on an air-space, water is passing through the 
cell-wall and is being evaporated, eventually diffusing through the open 
stoma into the atmosphere. 
Certain organic substances in the cell are being broken down, with the 
formation of carbon dioxide as a result. 
Carbon dioxide is being used by the chlorophyll corpuscles for the 
manufacture of starch grains or other carbohydrate. 
Enzymes are at work within the cell, transforming starch into sugar 
and effecting various other changes. ^ 
An exosmosis of elaborated products is taking place through the points 
of contact of the cell with other cells, which elaborated products eventually 
find their way into the sieve-tubes and are conducted to the growing 
regions or places of storage within the plant. 
At night or during darkness the stomata close, there is no diffusion of 
water vapour or other gases from the interior of the plant to the outer 
atmosphere, photosynthesis ceases, and the flow of water into the cell from 
the vessels stops or is reduced to a minimum. The conditions are therefore 
most favourable for the action of enzymes on the accumulated products of 
photosynthesis and the translocation of materials to the growing tissues of 
the plant. If the night temperature is sufficiently high, an extension in 
length of the stem would be the natural result. 
It is not claimed here that every relation of a plant to light is capable 
of explanation in this way, but only its growth during darkness. 
1 c. A plant normally flowers after it has attained a certain height and 
has stored up a quantity of reserve materials within its tissues. This is 
illustrated by biennial plants, bulbs, &c. A plant growing in dull light may 
have such a struggle for existence that it has no reserve material worth 
mentioning. As a result it will either not flower or will produce flowers 
very sparingly. Some trees in certain seasons produce such an abundance 
of flowers and fruits that it requires an interval of one or sometimes two 
years before the tree can accumulate sufficient reserve material for the 
formation of flowers again. 
As has been proved by the experiments, those plants subjected to 
a longer period of illumination grew faster and attained a greater weight as 
the result of prolonged photosynthesis. They would therefore naturally be 
in a position to bear flowers before those plants subjected to a shorter period 
of daylight. 
