776 PHYSIOLOGY. 



directly in a gaseous state by the leaves, and from the earth, 

 dissolved in water, by the roots. 



Oxygen is, next to carbon, the most abundant organic con- 

 stituent of plants ; and when we consider to what an enormous 

 extent it exists in nature, constituting as it does about 21 per 

 cent by volume of the atmosphere we breathe, eight ninths by 

 weight of the water we drink, and at least one lialf of the solid 

 materials around us and of the bodies of all living animals ; 

 we see that there are abundant materials from which plants can 

 obtain this necessary portion of their food. The whole of the 

 oxygen required by plants as food appears to be taken up by 

 them, either combined with hydi'ogen in the form of water, or 

 with carbon as carbonic acid. Some of the oxygen is therefore 

 obtained by the roots from the soil, and the other portion from 

 the air by the leaves. 



Hydrogen, the third organic constituent of plants, as just 

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

 form that plants obtain nearly the whole of this ingredient of 

 their food. It does not exist in a free state in tlie 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. 

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

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

 therefore, is taken by plants combined with oxygen as water, a 

 small portion is derived with nitrogen in the form of ammonia. 



Nitrogen, 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 there found. 

 Whether nitrogen can be taken up by plants in a free state is at 

 present doubtful (see p. 737), 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 phosphorus, which as we have noticed (p. 

 775) arc always combined with nitrogen in the protoplasmic 

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

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

 by the roots. 



In reviewing tlie sources of, and modes in Avhich, the different 

 organic constituents of plants are derived and taken up, wc see 



