ii.] A BUTTERCUP. 15 



be given off, under favourable circumstances, in a rapid 

 and continuous stream. These bubbles consist of almost 

 pure oxygen. 



This fixation of the carbon and liberation of the oxygen 

 of carbonic acid has been termed vegetable respiration j 

 but as the conditions which obtain are the reverse of 

 those characteristic of animal respiration, it may be more 

 correctly spoken of as characterising the process of 

 vegetable assimilation. Since the relative proportion of 

 oxygen in the products of assimilation is much less than 

 it is in carbonic acid and water, this liberation of the 

 oxygen in excess becomes a necessary concomitant in the 

 process. 



Repeated experiments have shown that some of the 

 parts of the flower, seeds when germinating, and also 

 plants or their organs which are not coloured green, 

 absorb oxygen from the air, and give off carbonic acid 

 gas. This may be regarded as a respiratory process, 

 corresponding precisely with that of animals, and accom- 

 panied by the liberation of sensible heat proportioned to 

 the intensity of the combustion. It is not improbable 

 that the green parts of plants also may, at least in the 

 dark, absorb oxygen and give off carbonic acid in the 

 same way, but in these parts the amount of carbon fixed 

 greatly predominates over that which is liberated. The 

 mutual relations, however, of these processes are as yet 

 but imperfectly understood. 



9. It is not yet perfectly clear from what source the plant 

 obtains its nitrogen ; not that the element is scarce, since 

 it forms four-fifths of the atmosphere, but the precise 

 state and combination (most probably as a nitrate) in 

 which it enters the plant, is still a matter of inquiry and 

 discussion among scientific men. It is not absorbed, as 

 free nitrogen, directly from the atmosphere, for purposes 

 of nutrition. 



10. The processes which we have briefly described of 

 absorption, transpiration, assimilation, and respiration, 

 we find, mainly concern the root, the stem, and the 

 leaves. These are the food-providers and preparers of 

 the plant. Hence we may speak of them collectively as 

 the Nutritive organs; the root being, as we have shown, 

 the organ specially of absorption, the foliage-leaves of 



