object and is enormously magnified. To the upper
orifice of the funnel is fixed a rubber tube, and by means of it steady
suction can be supplied. The result is that a current of air is drawn
through the stomata into the leaf, and then out of the leaf into the
cavity of the porometer. The rate of this current is an index of the
degree to which the stomata are open. With this apparatus a number of
interesting points can be determined.

[Picture: Fig. 8. Curve of Porometer readings in light and darkness
(black)]

Fig. 8 shows the effect of alternate periods of light and darkness. The
fall of the curve represents partial closure, and is seen to occur in the
periods of darkness (black), and to rise when the plant is re-illumined.
These changes are necessarily accompanied by rise and fall in the
evaporation of the leaf, but into the question of the accuracy of this
correlation I shall not enter.

There are other methods of demonstrating the movements of the stomata.
Stahl had the happy inspiration of making use of the colour-changes of
cobalt chloride. A piece of filter paper soaked in a 5 p.c. solution of
this salt is blue when dried, and turns pink in damp air. A dry piece of
this material, applied with proper precaution to the stomata-bearing
surface of a leaf, rapidly changes to pink if the stomata are open.
When, however, the same trial is made on the upper surface of a leaf,
where stomata do not occur, no such change occurs. If two leaves are
treated at the same time, one in the normal position and the other upside
down, it is delightful to watch the appearance of a pink picture of that
leaf whose stomatic surface is in contact with the paper, while no such
change takes place over that which exposes no stomata to the tell-tale
material. Another method was discovered by the accident of finding in an
old house in Wales a Chinese figure of a man, cut out of a thin shaving
of horn, which writhed and twisted when placed on the hand. It was
clearly very sensitive to moisture, and it seemed possible that
horn-shavings might be used to test the condition of the stomata. The
first difficulty was to obtain a supply of this material. Having
discovered from the P.O. Directory that there were two horn-pressers in
London I proceeded to visit one of them somewhere in Hoxton. He turned
out to be of a highly suspicious disposition, but his wife had more
discernment, and persuaded him t