198 THE FOOD AND NUTRITION OF PLANTS. 



under different circumstances, but it is always largest when the soil 

 is well supplied with manures that abound in nitrogen. The gluten 

 of wheat is a mixture of four isomei'ic quaternary products, distin- 

 guished by chemists under the names Fibrine (identical in nature 

 with that which forms the muscles of animals). Albumen (of the same 

 nature as animal albumen), Caseine (identical with the curd of milk), 

 and Ghitine. In beans and all kinds of pulse, or seeds of Legu- 

 minous plants, the azotized matter principally occurs in the form 

 of Legumine, which is nearly intermediate in character between albu- 

 men and caseine. 



358. Comparing now these principal products of assimilation in 

 plants with the inorganic materials from which they must needs be 

 formed, it may clearly be perceived that the principal result of vege- 

 tation, as concerns the atmosphere, from which plants draw their 

 food, consists in the withdrawal of water, of a little ammonia, and of 

 a large proportion of carbonic acid, and of the restoration of oxygen. 

 The latter is a constant effect of vegetation and the measure of its 

 amount. As respects the fabric of the plant, the sole consequences 

 of its formation upon the air are the withdrawal of a small quantity 

 of water, and of a large amount of carbonic acid gas, and the resto- 

 ration of the oxygen of the latter. In the formation of its azotized 

 materials, a portion of ammonia or of some equivalent compound of 

 nitrogen is also withdrawn. It is true, indeed, that leaves decom- 

 pose carbonic acid only in daylight ; and that they sometimes give a 

 quantity of carbonic acid to the air in the night, especially when 

 vegetation languishes, or even take from it a little oxygen. But 

 this does not affect the general result, nor require any qualification 

 of the general statement. The work simply ceases when light is 

 withdrawn. The plant is then merely in a passive state. Yet, 

 whenever exhalation from the leaves slowly continues in darkness, 

 the carbonic acid which the water holds necessarily flies off* with it, 

 during the interruption to vegetation, into the atmosphere from 

 which the plant took it. So much of the crude ssqt, or raw mate- 

 rial, merely runs to waste. Furthermore, it must be remembered 

 that the decomposition of carbonic acid in vegetation is in direct op- 

 position to ordinary chemical affinity ; or, in other words, that all 

 organized matter is in a state corresponding to that of unstable 

 equilibrium. Consequently, when light is withdrawn, ordinary 

 chemical forces may perhaps to some extent resume their sway, the 

 oxygen of the air combine with some of the newly deposited carbon 



