COAL MEASURES RENEATH ESSEX. I 35 



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 err^erge 

 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 grcnvth was greatly i?icreased as the percentage of carbonic 

 acid gas was higher in the air to zvhicli their foliage was exposed. I 

 was informed afterwards by one of the most distinguished botanists 

 in this country, that this result agreed v/ith 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. 



