THE RESPIRATION OF PLANTS. 63 



connected with an aspirator, which causes the air in contact with the 

 plant to pass gently over the baryta solution. For instance, take a 

 freshly-opened bud of the chestnut, and presently, or at least after a 

 very little while, there is seen to form, in the daylight, a deposit of 

 carbonate of baryta, and this increases very rapidly. Of course, care 

 must be taken to deprive the air of its carbonic acid before it is ad- 

 mitted into the bell-glass. 



A very simple experiment will make it plain that, in the course of 

 this first period, the nascent leaves absorb to an appreciable extent 

 the oxygen of the air both day and night. We have only to place 

 the plant in a small bell-glass containing common air, the mouth of 

 the vessel being stopped by means of a solution of caustic potash in a 

 saucer. Soon we observe the solution rising in the bell-glass, and 

 standing still at a certain point, which it never goes beyond. (Care 

 must be taken not to allow the alkaline liquid to touch the petioles of 

 the leaves.) If we now examine the elastic fluid which remains unab- 

 sorbed, we find that it contains nothing but nitrogen. In this opera- 

 tion the oxygen is inhaled by the leaves, which transform that gas 

 into carbonic acid ; this they expire in variable proportions according 

 to their age, and it is absorbed by the caustic-potash solution. 



But this power of absorbing oxygen and of exhaling carbonic acid 

 in the daytime, while very evident at the instant of the opening out 

 of the buds, becomes sensibly less pronounced, according as the leaves 

 grow, and, as a general rule, this phenomenon ceases to be presented 

 after these organs have attained their normal development. Hence, it 

 is certain that plants, in their earlier stages, respire after the manner 

 of animals, absorbing oxygen and exhaling carbonic acid. These 

 physiological facts were demonstrated by M. Corenwinder, in a me- 

 moir published in 1866 by the Societe des Sciences of Lille. 



III. 



It is not alone young plants just produced from the seed or from 

 the bud in the spring that offer these characters : all foliaceous or- 

 gans, while young, tender, and injected with nitrogenized materials, 

 and just beginning to derive their nourishment from the carbon of the 

 atmosphere, sensibly exhale carbonic acid in the daytime. If we ob- 

 serve the young branches which, during summer, grow on trees of 

 persistent foliage, the Laurocerasus, for example, we find that, in these 

 new growths, the phenomenon of respiration predominates : they ex- 

 hale sensibly carbonic acid in the daytime. 



But, if we place under a closed bell-glass an entire branch bearing 

 leaves of the current and of the preceding year, collecting the air that 

 has been in contact with them in a receiver holding baryta-water, and 

 provided with an aspirator, we find that the result varies according to 

 the relative quantities of new and old leaves. If the latter are in ex- 

 cess, they absorb the carbonic acid exhaled by the former, and the 



