to
suck up the water it needs through a coarse thermometer tube, dipping
into a beaker. The laurel does not wither, and we know therefore that it
is continuously supplied with water. If the beaker is removed we shall
see the absorption, for the thermometer tube does not remain full of
water; a minute column of air is seen at its lower end which rapidly
increases in size, and finally when the tube is emptied of its
water-content, bubbles of air escape one after another into the larger
tube, which contains the cut end of the branch. This, the simplest
possible experiment, is nevertheless a vivid ocular proof of the laurel's
power of absorbing water. It can be shown that the sucking power of the
branch depends on its leaves, for if these are removed the rate of the
current is very greatly diminished. It can also be proved that it
depends on some quality of the leaf surface, for if a new specimen is
taken, and if the lower sides of its leaves are rubbed with vaseline, the
rate of absorption will be seen to diminish very greatly. Greasing the
upper surface of the leaves does not produce this result, and when we
examine the two surfaces it is found that the lower one is riddled with
innumerable microscopic holes (stomata), while the upper side of the leaf
has no such apertures. The stomata in fact are the arbiters of what
shall pass in or out of the body of the leaf; they are the gate-keepers
who regulate both export and import. They are known by actual inspection
(with a microscope) to close at night: the result of this is that the
evaporation of the leaves is much slower at night, and this is true when
allowance has been made for the fact that evaporation is also checked at
night by the dampness of the air.

[Picture: Fig. 7. The Porometer]

The microscopical inspection of stomata is not a completely satisfactory
method of discovering to what degree they are open. It has, however,
been my good fortune to resuscitate and simplify a method of studying the
stomatal condition. The method was many years ago tried in a hopelessly
cumbersome form by a German, but never came into use. My apparatus is
described in the _Proceedings of the Royal Society_, {212} and is known
as the Porometer. Its essential part is shown in Fig. 7. It consists of
a funnel-shaped tube, having a broad flange, which is cemented on to the
stomata-bearing surface of a leaf. The leaf is represented by the
obliquely shaded