40 EVOLUTION AND GROWTH. 



If a root detaohed from the stem be introduced under a bell-glass 

 full of oxygen gas, the volume of the gas diminishes, carbonic acid 

 is formed, of which a portion only mingles with the gas of the 

 receiver, a certain quantity being retained by the moisture of the 

 root. The volume of the gas thus retained is always less than that 

 of the root itself, however long the experiment may be continued. 

 In these circumstances, whether in the shade oi the sun, roots act 

 precisely as leaves do when kept in the dark. Roots still connected 

 with their stems, give somewhat different results. 



When the experiment is made with the stem and the leaves in the 

 free air, while the roots are in a limited atmosphere of oxygen, they 

 then absorb several times their own volume of this gas. Tliis is be- 

 cause the carbonic acid formed and absorbed is carried into the general 

 system of the plant, where it is elaborated by the leaves, if exposed to 

 the same light, or simply exhaled if the plant be kept in the dark. 



The presence of oxygen in the air which has access to the roots 

 is not merely favorable ; it is absolutely indispensable to the exer- 

 cise of their functions. A plant, the stem and leaves of which are 

 in the air, soon dies if its roots are in contact with pure carbonic 

 acid, with hydrogen gas, or azote. The use of oxygen in the growth 

 of the subterraneous parts of plants, explains wherefore our annual 

 plants, which have largely developed roots, require a friaWe and 

 loose soil for their advantageous cultivation. This also enables us 

 to understand wherefore trees die, when their roots are submerged 

 in stagnant water, and wherefore the effect of submersion in general 

 is less injurious when the water is running, such water always con- 

 taining more air in solution than that which is stagnant. 



The woody parts, the fruit, and those organs of plants in general 

 which have not a green color, stand in the same relations to oxygen 

 as the roots : they merely change th-s gas into carbonic acid, which 

 is then transported to the plant at large, to suffer decomposition by 

 the green parts. In this action we observe a displacement, a kind 

 of translation of the carbon of the lower to the upper parts of 

 plants. 



The decomposition of carbonic acid by j)lants admitted, we have 

 still to examine whether, in the phenomena of vegetation, the leaves 

 decompose the carbonic acid of the atmosphere directly, or the acid 

 gas, previously dissolved in the water, which m«)istens the ground, 

 be conducted by the way of absorption into the tissues of vegetables, 

 there to suffer decomp«)sition. The quantity of carbonic acid con- 

 tained in the air is so small, and the growth of plants, on the con- 

 trary, is often so rapid, that it might reasonably be suspected that 

 the carbon which they require was introduced in great part by this 

 way of absorption. In that series of beautiful experiments in which 

 M. Saussure exp<ised plants to the influence of atmospheres more 

 or less charged with carbonic acid, the water in which their roots 

 were plunged was in contact with the mixed atmospheres. It was 

 therefore possible that the carbonic acid gas entered the vegetables 

 m the solution by the roots. 



Sennehier made an experiment t<» show that leaves doo«»miK)»« 



