180 THE FOOD AND NUTRITION OP PLANTS. 



table substance, are termed the universal, organic constituents of 

 plants. They are Carbon, Hydrogen, Oxygen, and Nitrogen (10, 

 27). The proper vegetable structure, that is, the tissue itself, 

 consists of only three of these elements, namely, carbon, hydrogen, 

 and oxygen ; while the fourth, nitrogen, is an essential constituent 

 of the protoplasm, which plays so important a part in the formation 

 of the cells and is an element of one class of vegetable products. 



323. The Organic Constituents. These four elements mu>t be fur- 

 nished by the food upon which the vegetable lives ; ■ — they must 

 be drawn from the soil and the air ; in some cases, doubtless, from 

 the latter source, as in Epiphytes, or Air-plants (149), but gener- 

 ally and principally by absorption through the roots. The plant's 

 nourishment is wliolly received either in the gaseous or the liquid 

 form ; for the leaves can imbibe air or vapor only, and the roots are 

 incapable of taking in particles of solid matter, however minutely di- 

 vided (40, 133). 



324. In whatever mode imbibed, evidently the main vehicle of 

 the plant's nourishment is water, which as a liquid or as vapor is 

 continually in contact with its roots, and in the state of vapor always 

 surrounds its leaves. We have seen how copiously water is taken 

 up by the growing plant, and have formed some general idea of its 

 amount by the quantity that is exhaled unconsumed by the leaves 

 (313). But pure water, although indispensable, is insufficient for 

 the nourishment of plants. It consists of oxygen and h3drogen ; 

 and therefore may furnish, and doubtless does principally furnish, 

 these two essential elements of the vegetable structure. But it can- 

 not supply what it does not itself contain, namely, the carbon and 

 nitrogen which the plant also requires. 



325. Yet the question arises, whether the water which the plant 

 actually imbibes contains in fact a quantity of these remaining 

 elements. Though pure A\'ater cannot, may not rain-ivater supply 

 the needful carbon and nitrogen ? It is evident that, if the water 

 which in such large quantities rises through the plant, and is ex- 

 haled from its leaves, contain even a very minute quantity of these 

 ingredients, in such a form that they may be detained when the 

 superfluous water is exhaled, this might furnish the whole organic 

 food of the vegetable ; since the plant may condense and accumu- 

 late the carbon and nitrogen, just as the extremely minute quantity 

 of earthy matter which the water contains is in time largely accu- 

 mulated in the leaves and wood. 



