504 ORGANOLOGY. 



the light of the sun. Boussingault enclosed the branch of a vine in a 

 receiver, through which, by means of an aspirator, air containing a 

 known quantity of carbonic acid was admitted, and after being exposed 

 to the action of the plant, the quantity of carbonic acid was measured 

 by means of an alkali ; whilst at the same time the same air, without 

 having come into contact with the plant, proved its carbonic acid contents. 

 The result was, that half the carbonic acid of the air was absorbed by 

 the plant. The absolute quantity of the air which, in a definite time, 

 came into contact with the plant, and also the absolute quantity of car- 

 bonic acid, were not measured in this experiment. 



I must here also object even to the most accurate experiments of 

 De Saussure. Plants vegetating in the light absorb carbonic acid and 

 exhale oxygen gas ; but the quantities of oxygen and of carbonic acid 

 do not stand in any equal relation to each other in his experiments. The 

 following plants, instead of absorbing an equal volume of 10 cubic cent- 

 ners of carbonic acid gas for each 100 cubic centners of oxygen gas which 

 they exhale, absorbed the following quantities of carbonic acid : 



Vinca minor . . 147'6 cubic centners. 



Mentha aquatica . . 137*2 



Lyihrum Salicaria . . 123*1 



Pinus genevensis . . 123-6 



Of the entire amount of oxygen received, they retain from J to ^. 



The facts which have heretofore been recorded lie as yet unexplained 

 before us. There is no present possibility of bringing them into harmony 

 with our physical knowledge, for both the one and the other are in an in- 

 complete condition. Our next effort must be to pursue those phenomena 

 which we find exhibited in certain definite groups of cells, or which are 

 manifested within certain determinate portions of time, in order to learn 

 to separate them from what belongs unceasingly to the vegetation of each 

 individual cell. 



III. Assimilation of Food. 



201. The principal substances absorbed by plants are water, 

 carbonic acid, carbonate of ammonia, and certain inorganic salts. 

 The plant draws all these from the soil by the spongioles. It re- 

 ceives carbonic acid from the air through the leaves. In what 

 relation these two methods of absorption stand to each other, and 

 to the necessities of the plant, is unknown to us. The exhalation 

 and the interchange of gases is carried on in connection with the 

 air immediately in contact with the surface of the plant, and also 

 between the cellular tissue and the intercellular passages, from 

 whence gas and vapour escape through the stomates. 



As the great mass of substances which form the plant contain 

 less oxygen than the plant absorbs, it follows of necessity that 

 during the process of assimilation oxygen is set free. Probably 

 neither oxygen nor water are decomposed directly, but a series 

 of combinations takes place, from which, at certain points, or 

 else finally, oxygen is set free. For example, a small portion of 

 the oxygen in the green parts of plants appears to originate in a 



