FOOD OF pla>;ts and its sources. 793 



Hydrogen, the third organic constituent of plants, as jnst 

 noticed, forms one-ninth by weight of water, and it is in this 

 form that phmts obtain nearly the whole of this ingredient of 

 their food. It does not exist in a free state in the atmosphere 

 nor in the soil, and hence cannot be obtained by plants in a 

 simple state. In combination, however, with nitrogen, it forms 

 ammonia, which always exists to some extent in the atmosphere 

 and in the excretions of animals ; and is also always produced 

 during the decomposition of animal matter. Ammonia exists 

 in a gaseous state in the atmosphere, and being freely soluble 

 in water, the rain as it passes through the air dissolves it, and 

 carries it down to the roots, by which organs it is taken up. 

 Tlie roots in like manner absorb the ammonia which is con- 

 tained in the soil. While the larger proportion of hydrogen, 

 therefore, is taken up combined with oxygen as water, a small 

 portion is acquired with nitrogen in the form of ammonia. 



Isitrogen, the fourth and last organic constituent of plants, 

 constitutes about 79 per cent, of the volume of the atmosphere, 

 and is an important ingredient in animal tissues. It also exists 

 in combination with oxygen as nitric acid in rain water, and in 

 the soil as a constituent of the various nitrates and animal pro- 

 ducts there found. Whether nitrogen can be taken up by plants 

 in a free state is at present doubtful (see p. 751), but it is quite 

 clear that the principal form in which it is absorbed is as 

 ammonia. Some believe that a small part is obtained from 

 nitric acid and nitrates. 



Both sulphur and j^hosphonis, which as we have noticed (p. 

 791) are always combined with nitrogen in the protoplasmic 

 cell-contents, are obtained in a state of combination from the 

 soil. They are dissolved in the water, and are thus absorbed by 

 the roots. 



In reviewing the sources of, and modes in which, the different 

 organic or volatile constituents of plants are derived and taken up, 

 W8 see that the sources are the earth and the air, more particu- 

 larly the latter ; and that they are principally absorbed in the 

 forms of carbonic acid and water, the latter of which is not only 

 food in itself, as it is composed of oxygen and hydrogen, two of 

 the essential organic constituents of plants, but it is also the 

 vehicle by which other food is conveyed to them. 



2. The Inorganic Constituents or Ash, and their Sotirces. — The 

 amount of inorganic matter found in plants, as already observed 

 (p. 791), is very much less than that of the organic. The 

 inorganic matters are all derived from the earth in a state of 

 solution in water which contains carbonic acid, and hence we 

 see again, how important a proper supply of water is to plants. 

 While the organic constituents are the same for all plants, the 

 inorganic constituents vary very much in different plants. The 

 inorganic constituents differ from the organic also, in the follow- 



