184 THE FOOD AND NUTRITION OF PLANTS. 



such as a piece of wood, after the incrusting mineral matter has 

 been chemically removed, invariably yields but three or four ele- 

 ments. These, which are indispensable to vegetation, and make 

 up at least from eighty-eight to ninety-nine per cent, of every veg- 

 etable substance, are termed the universal, organic constituents of 

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

 The proper vegetable structure, that is, the tissue itself, uniformly 

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

 and oxygen. These are absolutely essential and universal ; while 

 the fourth, nitrogen, is an essential constituent of the protoplasm, 

 which plays so important a part in the formation of the cells (27), 

 and of certain vegetable products. 



323. The Organic Constituents, These four elements must 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 (132), but gener- 

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

 nourishment is wholly received either in the gaseous or the liquid 

 form; for the leaves can imbibe air or vapor only (262-268), 

 while the tissue of the rootlets is especially adapted to absorb 

 liquids, and is incapable of taking in solid matter, however mi- 

 nutely divided (Fig. 108-110). 



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

 the plant's nourishment is water, which as a liquid bathes its roots, 

 and in the state of vapor continually 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 hydrogen ; and therefore may furnish, 

 and doubtless does principally furnish, these two essential elements 

 of the vegetable structure. But it cannot 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 water cannot, may not rain-water 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 



