THE VEGETABLE CELL. 87 



oxygen gas upon vegetable substance, be interrupted, tlie plant, 

 just like an animal, becomes asphyxiated, and death, follows 

 quickly. If we wish to speak of a respiration in plants, this 

 oxygen-consuming breathing deserves the name far more than 

 the exhalation of oxygen by the green organs, connected with the 

 nutrient processes. In this immediate relation to life the respira- 

 tion of plants corresponds completely with the respiration of ani- 

 mals ; oxygen gas is a true vital air to plants. But the behaviour 

 of the plant towards the atmosphere becomes the more compli- 

 cated, that it does not merely absorb oxygen from without, like 

 the animal, but also a part of that prepared in its own green 

 organs. 



Obser^. Liebig must shut Ms eyes to facts lying open before Mm, wlien 

 lie persists {^^Agricultural Chemistry,'' 6th ed.) that the respu'aiioa consum- 

 ing oxygen does not exist, that the absorption of oxygen has notMng to 

 do with the life of plants, but is a process of oxidation, which occurs in 

 dead wood as in the hvmg plant, and that the exhalation of caibonic acid 

 stands m no connexion with the absorption of oxygen, but that the car- 

 bonic acid simply rises in the stem with the water taken up by the roots, 

 as m a cotton wick, and so passes out into the air. 



Although the great diffusion of water and carbonic acid almost 

 everywhere give full opportunity to plants, of appropiiating the 

 three principal elements of their substance (carbon, hydrogen, and 

 oxygen), they have not always the opportunity of absorbing the 

 quantity of nitrogen requisite for a vigorous development, whence 

 the important part which nitrogenous substances play in manur- 

 ing. The nitrogen of the air is a perfectly indifferent body to- 

 wards plants. Even Saussure indicated that plants can only take 

 up nitrogen in the form of solutions of organic substances or of 

 ammonia ; the latter has been especially maintained by Liebig, 

 and his was the merit of demonstrating by experiment that ammo- 

 niacal vapours exist in atmospheric air, and that ammonia occurs 

 in all rain and snow water ; and on the other hand, of directing 

 attention to the presence of abundance of ammoniacal salts in the 

 ascending sap of the Maple, Birch, &;c. "Whether, however, as 

 Liebig assumes, the ammonia contained in atmospheric air suffices 

 to furnish the nitrogen contained in wild plants, and that^an 

 abundant supply of ammonia from the soil is necessary to culti- 

 vated plants, only because it is desired to stimulate them to the 

 production of a great mass of the constituents of blood, is quite a 

 different question. In the first place, no experiment has shewn 

 that plants are capable of applying to their nutrition the ammo- 

 niacal vapours contained in the atmosphere ; secondly, it is even 

 doubtful whether this is the case with the ammoniacal salts which 

 they take up by their roots, for, according to Bouchardat Q' Re- 

 cherches sur la Vegetation,'' 24), these salts, when absorbed by 

 plants in watery solutions are poisonous to them in a state of 

 1000 or 1500 fold dilution. But it is proved by abundant expe- 



