i 4 EXAMINATION OF [CHAP. 



gen. And these elements are present, there is reason to 

 believe, in every organized being, whether plant or animal, 

 in combinations peculiar to organized beings. Hence, 

 they may be called the organic elements, in contradistinc- 

 tion to the (mineral) elements found in combinations which 

 are not peculiar to organized beings, and several of which 

 remain in the ash of plants when burnt. The latter may 

 be called the inorganic elements. 



6. These four organic elements do not exist separately 

 in the plant, but, as we have said, in combination. Thus 

 the carbon is united with oxygen and hydrogen (the last 

 two the elements of water), forming the basis of a series 

 of compounds, called ternary compounds, because they 

 are composed of three elements. The nitrogen, with the 

 addition of sulphur, occurs combined with the same three 

 elements, forming a more complex compound, called 

 protoplasm, the medium through which all phenomena 

 of vitality are primarily manifested. And these two series 

 of organic compounds stand in remarkable contrast to 

 each other in the plant, both in respect of the structures 

 in which they respectively take more or less prominent 

 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 certain 

 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 combina- 

 tion with oxygen, as carbonic acid gas, which occurs as a 

 constant constituent of the atmosphere ; and the green- 

 coloured organs of plants, under the influence of sunlight, 

 possess the power of abstracting 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 combination with the carbon of the carbonic 

 acid gas. This liberation of oxygen is most easily shown 

 by taking a few 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 



