480 ME. J. R. LAWKS, DB. (ill.HLKT, AND QB. PUOH OS 



in the open rii «-. and were protected from ruin, or the too powerful rays of the sun, bj 

 canvas owning which could be drawn down over them, or withdrawn, at pleasure. 



In 1858, in addition to the sets of apparatus above described, two glazed i ach 



as were used by M. (i. Yn.i.r: in his experiments, and which he kindly sent over to us 

 lor the purpose, were employed 



J. — The supply of Carbonic Add to the Plants. 



Owing to the small proportion of carbonic acid in the atmosphere, and to the fact 

 that a part of it would be absorbed in passing through the apparatus just described, it 

 was necessary to give a supply of it to the plants artificially. It was obtained by the 

 action of chlorhydric acid upon fragments of marble in the vessel T, Plate XIII., or I", 

 fig. 1, Plate XIV. 



In regulating the supply of carbonic acid, the points to be observed were, to keep the 

 proportion in the enclosed atmosphere below that in which it would prove injurious to 

 the plants, and at the same time to provide a sufficient quantity for the demands of 

 vegetation at the most active periods of growth. 



BorssixiiAULT found* that the air surrounding a plant might, consistently with 

 healthy growth, contain 8 per cent, of carbonic acid. This amount, then, on the one 

 hand, and the very small quantity in the atmosphere which is sufficient for natural 

 vegetation (about 0-04 per cent.), on the other hand, afford us limits between which a 

 wide range is allowed for variation. 



Calculation showed that a minimum quantity of 02 per cent, of carbonic acid in the 

 air of the enclosing apparatus would supply 5 cubic inches of the gas within the shade 

 at one time, corresponding to 0-0439 gramme of carbon — a quantity which, maintained 

 daily throughout the sunlight, would be very much more than was required by the plants. 



It is obvious, therefore, that a variation in the amount of carbonic acid in the atmo- 

 sphere of the plants between 4*0 per cent, and 0-2 per cent, would be very safely within 

 the limit suggested by the experiment of Boussixgault as the maximum, on the one 

 hand, and that indicated by the above considerations as the minimum desirable in the 

 experiment, on the other. 



A question arises as to the influence which the changes in the proportion of carbonic 

 acid in the air, between the assumed limits, may have upon the plant. In reference to 

 this point, it may be mentioned that our own experiments upon the nature of the gas 

 in plants (some of the results of which will be given further on) appear to show that 

 the changes in the proportion of the carbonic acid in the air of the cells and intercellular 

 passages, and in that in the fluids of the stem, are much greater, and more rapid than 

 those which can take place in the atmosphere of our apparatus. In addition to this, 

 may be stated the fact that plants derive much of their carbonic acid from aqueous 

 solution absorbed by the roots; and most probably the remainder is absorbed by the 

 fluids of the plant before influencing its growth. These absorptions can take place but 

 * Memoires de Chirnie Agricole et de Physiologie, 1S54. p. 441. 



