COAL MEASURES BENEATH ESSEX. 135
peat-deposits of the globe, together with all the carbon which forms
the essential element of the limestones of the subsequently-formed
strata of the crust of the earth, existed as free carbonic acid gas,
either suspended in the atmosphere or dissolved in the waters of the
universal ocean, at the time when the land first began to emerge
from the ocean. This carbonic acid gas is—as is well known—
the essential food-stuff of the plant world ; and we should expect,
therefore, that when once a land vegetation had got a start, as we
have noticed above, in the Devonian Age, it would develop and
multiply with a vigour and rapidity altogether unknown to us in the
present stage of the earth's history, in which the vegetation of the
globe may be said by comparison to subsist upon "starvation diet."
This has, I know, been called in question by some whose opinions
are entitled to respect ; and several years ago the question was raised
again in the pages of the "Geological Magazine." This led me in
the summer of 1888 to carry out a series of experiments on the effect
of atmospheres of different compositions with varying proportions of
carbonic acid upon plants of the same kind, of the same age and
healthy growth, exposed to the same conditions of light and tempera-
ture. The general results, which were published in the "Report of
the British Association Meeting" at Bath in that year, showed
clearly enough that so long as the roots of the plants were kept well
supplied with water, and there was a fair percentage of free oxygen
in the atmosphere to maintain the activity of the protoplasm, the
rapidity of growth was greatly increased as the percentage of carbonic
acid gas was higher in the air to which their foliage was exposed. I
was informed afterwards by one of the most distinguished botanists
in this country, that this result agreed with the results which had
been recently obtained in a similar way in Germany. As I pointed
out in the "Report" referred to (page 661) this great and extra-
ordinary development of plant life (chiefly Vascular Cryptogams,
Conifers, and such intermediate forms as Sigillaria) in the Carbon-
iferous Age served both "as a means of storage of carbon in the
earth's lithosphere and to purify the atmosphere, so as to render it fit
for the development of air-breathing forms of life in the Mesozoic
Age." There can be no doubt that with the removal of carbonic acid
from the air the supply of oxygen to it was proportionately increased;
assimilation of carbon by the green colouring matter of plants under
the stimulus of sunlight and the setting free of oxygen being two
concomitant factors of the fundamental law of vegetable growth.