64 THE POPULAR SCIENCE MONTHLY. 



baryta-water remains clear, but it grows turbid when the new leaves 

 predominate. 



If the experiment be made at the period when all the leaves of the 

 current year have attained their adult age, the branch of Laurocerasus 

 gives out no carbonic acid while exposed to light, provided the light 

 is not very feeble. 



The point at which plants cease perceptibly to give out carbonic 

 acid in the daytime varies widely according to the species. Coren- 

 winder has found some which exhibit this property for a long time, 

 while others lose it very early. In the first category we may class a 

 perennial plant common in our gardens, Diclitra spectabilis, and in 

 the second the young leaves of the beet. 



The cause of this peculiarity cannot at present be assigned ; certain 

 it is, however, that it largely depends on external circumstances, heat, 

 for instance, which quickens all the chemical actions of oxygen, or the 

 intensity of the light which promotes the assimilation of the carbon. 

 But the special nature of the plant also plays a part. Hence we must 

 not jump at conclusions after one of these experiments, if we would 

 avoid setting up artificial laws with many exceptions. 



It was at first difficult to account a priori for the fact of this prop- 

 erty of nascent plants constantly exhaling carbonic acid, being at the 

 outset very patent, and then diminishing in intensity as they grow, 

 and finally disappearing. But experiments of another kind, described 

 eight years ago by M. Corenwinder, put him on the track of this phe- 

 nomenon, and gave him a plausible explanation of it. 



Adopting the same processes which enabled Bonnet, Ingenhousz, 

 and Sennebier, to lay the foundations of plant physiology, he placed 

 buds and young stems bearing new leaves in bell-glasses filled with 

 spring-water containing bicarbonate of lime or in water charged with 

 carbonic acid, and then exposed them to the sun. As was to be ex- 

 pected, the leaves were soon covered with bubbles, and gave off oxygen ; 

 and this is the case even with leaves whose evolution is not yet complete. 

 Hence it is plain that, from the earliest period of their life, plants de- 

 compose the carbonic acid of the atmosphere and assimilate its carbon. 



Thus the foregoing experiments prove two facts which seem to be 

 contradictory, and which, nevertheless, are simultaneous: I. Inhala- 

 tion of oxygen, accompanied with emission of carbonic acid ; 2. Ab- 

 sorption of carbonic acid, leading to a discharge of oxygen. Hence, 

 in young plants, there is simultaneity of the two modes of respiration 

 commonly attributed to older plants; but, in the latter, these two 

 modes have different conditions or different organs. This was the 

 starting-point, and it had to be made clear by means of accurate re- 

 search. 



IV. 



As we now see, the plant begins, in the early stages of its life, to 

 respire as the animal does, absorbing oxygen, and exhaling carbonic 



