II.] ASSIMILATION. 21 



both in respect of the structures in wliich they respectively 

 take part, and of function, as we shall point out when we 

 come to speak of the minute structure of plants. 



7. We have already explained how water (oxygen and 

 hydrogen) finds access to the plant, as well as the mineral 

 substances which may be held in solution by the water. 

 With regard to the important element carbon, experiments 

 clearly show that it is absorbed in combination with oxygen, 

 as carbonic acid gas, which is soluble in water, and may 

 thus be taken up by the root. Carbonic acid gas also 

 occurs in the atmosphere, and green leaves, under the influ- 

 ence of direct sunlight, possess the power of absorbing it 

 directly from the air. 



8. But the most remarkable circumstance attending this 

 absorption of carbonic acid is the liberation of oxygen gas 

 by the leaves, very nearly to the amount absorbed in com- 

 bination with the carbon of the carbonic acid gas. This 

 liberation of oxygen is most easily shown by taking a it.\^ 

 leaves which have been first soaked a day or two in water 

 so as to become saturated, and exposing them, plunged in 

 water containing carbonic acid (as ordinary spring or pump 

 water, in which it is always present), to direct sunlight. 

 Minute bubbles will be given off, under favourable circum- 

 stances, 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 respiratio7i^ but 

 as the conditions which obtain are the reverse of those 

 characteristic of animal respiration, it may be more correctly 

 spoken of as a part of the process of vegetable assimilation. 



Repeated experiments have shown that some of the parts 

 of the flower, seeds when germinating, and also plants 

 which are not coloured creen. absorb oxygen from tlie air, 



