ways pleasant to see the
leaf that had been supplied with CO2 turn blue, while the starved leaf
remains a hungry yellow.

Some of the prettiest methods of demonstrating this process depend, not
on the manufacture of starch in the leaf, but on the fact that an
assimilating plant sets free oxygen, by breaking up the molecule CO2,
building the carbon (C) into its own tissues, and letting the oxygen (O)
go free. A beautiful method was discovered on these lines by Engelmann,
which I was never tired of seeing year after year in my Cambridge class.
Defibrinated bullock's blood is freed from air by means of an air pump
and charged with CO2. In the course of this process it acquires the
dingy tint of venous blood. A single leaf of the American weed (Elodea)
is mounted on a glass slide in a drop of this blood and covered by an
ordinary cover slip. Then comes the dramatic moment: the preparation is
exposed to sunshine, and in 3 or 4 minutes a delicate scarlet border
begins to appear round the leaf and grows rapidly, making a curious
sunset effect in contrast with dingy purple of the venous blood. The
meaning is very clear; the Elodea leaf in sunshine took the carbon from
the CO2, and the oxygen thus set free gave the venous blood the scarlet
hue characteristic of the arterial condition. Professor Farmer has
designed a striking method based on another well-known experiment of
Engelmann's. A drop of water containing the products of decay, and
therefore swarming with bacteria, supplies the test. A drop of this
fluid is placed on a glass slip, one or two delicate leaves of a green
water plant (Elodea) are added, and a square of thin glass is placed on
it. Round the edges of the cover-slip the preparation must be sealed
with a preparation of wax, which melts at a low temperature, and when
cold serves to prevent the preparation drying; it also isolates it from
the surrounding atmosphere. After making sure under the microscope that
the bacteria are in active movement, the glass slip is placed in the dark
for some 3 or 4 hours. It is then examined, and the bacteria will be
found to have ceased to move because they and the leaves between them
have consumed the oxygen dissolved in the water, and bacterial activity
being dependent on oxygen naturally came to an end. The preparation is
placed under the microscope and illumined with bright incandescent gas,
and after a short time the bacteria begin to stir and are soon once more
wh