hat I was a harmless customer, with no
designs on trade secrets, and I finally obtained what I wanted. A
delicate strip of horn was fixed to a little block of cork and placed on
a leaf, and to my delight showed the stomata to be open by violently
curving upwards. It was only necessary to fix a graduated arc to the
cork, and to fasten a delicate hair on to the horn so as to serve as
index. The instrument is not of course accurately quantitative, but it
does at least show whether the stomata are nearly shut, moderately open,
or widely so. Rough as it is I found it good enough for determining a
number of interesting facts in the physiology of stomata. {215a}

I now pass on to a different subject, the all-important process on which
the life of green plants depends, an act therefore by which our own
existence and that of all other animals is conditioned. I mean the
process known as _assimilation_. This is the truly miraculous feat of
using as a source of food the carbonic acid gas (CO2) which exists in
minute quantities in the atmosphere. The plant is in fact a
carbon-catching machine, and the machine is driven by the energy of the
sun, and can therefore only work in light. The eminent Russian botanist,
Timiriazeff, in a lecture on this subject {215b} before the Royal
Society, made a witty use of _Gulliver's Travels_--a book not commonly
quoted as an authority in scientific matters. He pointed out that the
philosophers of Lagado, who were extracting sun-beams from cucumbers,
were not doing anything absurd. On the contrary, since the cucumbers had
been built with the help of sunshine, it was a reasonable expectation
that energy corresponding to the sunshine should be obtainable. This
indeed is what we do when we drive a steam engine by burning coal which
ages ago was built by vegetable machinery driven by sunlight.

It is possible to show the existence of this process by very simple
experiments. The most direct, but the least interesting, experiment is
to take two similar plants, and expose plant _A_ to an atmosphere
containing CO2 while _B_ is in air freed from that gas. Both specimens
are placed in bright light, and after a sufficient interval of time their
leaves are tested for the presence of starch. This is a simple matter;
the green colouring matter is washed out of them by means of alcohol, and
they are then placed in a dilute solution of iodine, which has the
property of staining starch purple. It is al