DEC03IP0SITI0N OF CARBONIC ACID. 39 



Saussure applied the names of inspiration and expiration of plants 

 to these alternate effects, led by the analog^y — somewhat remote, it 

 must be confessed — which the phenomenon presents with the respi- 

 ration of animals. 



The inspiration of leaves has certain limits ; in prolonging their 

 stay in the dark, the absorption becomes less and less : it ceases 

 entirely when the leaves have condensed about their own volume of 

 oxygen gas. And let it not be supposed that the nocturnal inspira- 

 tion of leaves is the consequence of a merely mechanical action, 

 comparable, for example, to that exerted by porous substances gen- 

 erally upon gases. The proof that it is not so is supplied by the 

 fact that the same effects do not follow when leaves are immersed in 

 carbonic acid, hydrogen, or azote. In such circumstances there is 

 no appreciable diminution of the atmosphere that surrounds* the 

 plant. The primary cause of the inspiration of oxygen by the 

 leaves of living plants is, therefore, obviously of a chemical nature. 



With the facts which have just been announced before us, it seems 

 very probable that during the nocturnal inspiration, the carbonic acid 

 which appears is formed at the cost of carbon contained in the leaves, 

 and that this acid is retained either wholly or in part, in proportion 

 as the parenchyma of the leaf is more or less plentifully provided 

 with water. A plant that remains permanently in a dark place, 

 exposed to the open air, loses carbon incessantly ; the oxygen of the 

 atmosphere then exerts an action that only terminates with the life 

 of the plant : a result which is apparently in opposition to what takes 

 place in an atmosphere of limited extent. But it is so, because in 

 the free air the green parts of vegetables can never become entirely 

 saturated with carbonic acid, inasmuch as there is a ceaseless 

 interchange going on between this gas, and the mass of the surround- 

 ing atmosphere ; there is, then, incessant penetration of the gases, 

 as it is called. There is a kind of slow combustion of the carbon 

 of a plant which is abstracted from the reparative influence of the 

 light. 



The oxygen of the air also acts, but much less energetically, upon 

 the organs of plants that do not possess a green color. 



The roots buried in the ground are still subjected to the action of 

 tiiis gas. It is indeed well known, that to do their office properly, 

 the soil must be soft and permeable, whence the repeated hoeings 

 and turnings of the soil, and the pains that are taken to give access 

 to the air into the ground in so many of the operations of agricul- 

 ture. The roots that penetrate to a great depth, such as those of 

 many trees, are no less dependent on the same thing ; the moisture 

 that reaches them from without brings them the oxygen in solution, 

 which they require for their development. It is long since Dr. 

 Stephen Hales showed that the interstices of vegetable earth still 

 contained air mingled with a very considerable proportion of oxygen. 

 The roots of vegetables, moreover, appear generally to be stronger 

 and more numerous as they are nearer the surface. In tropical 

 countries various plants have creeping roots which often acquire 

 dimensions little short of those of the trunk they feed. 



