CHEMICAL BASIS OF PLANT FORMS 235 



and moisture. The air which surrounds the leaves of land plants 

 is a mixture of nitrogen and oxygen with small quantities of 

 carbon-dioxide, ammonia, varying quantities of aqueous vapor, 

 and occasionally traces of nitric acid. 



The elements from these media are absorbed by different 

 parts of the plant, and there is a difference in the manner of 

 absorption by fungi, parasites, air plants, and green plants. 

 However, the elements which are absolutely essential for the 

 nutrition and maintenance of the life of all plants are carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, iron, in the case of green plants, and, in 

 certain cases, chlorine. 



It is characteristic of plants that they must absorb their food 

 in the fluid form. The absorbent organs of plants are the roots, 

 for water and salts in solution, and the leaves, for gases. In the 

 lower plants, where there are no roots or leaves, water or sub- 

 stances in solution and gases are absorbed either directly by 

 the cells of the thallus, or by root hairs. Among the higher 

 plants, the root hairs and the uncuticularized epidermal cells 

 of the younger roots are active in absorbing material from the 

 soil. Any part of the plant, if immersed in water, will absorb a 

 smaller or a larger quantity of it; as, for instance, cut flowers 

 placed with their cut ends in water will absorb, for a time, suffi- 

 cient to prevent withering. The absorption of gases in higher 

 plants is by means of the leaves, for it has been found that car- 

 bon-dioxide is absorbed from the air by those organs which are 

 green and contain chlorophyll ; and in experiments where the 

 carbon- dioxide of the air was cut off from the leaves, though it 

 was supplied to the roots, it was found that the plant could not' 

 live long. It has also been found that the presence of carbon- 

 dioxide in another part of the plant does not contribute to the 

 formation of starch in the leaves. 



Green plants obtain their carbon from the carbon-dioxide of 

 the air. Plants which do not contain chlorophyll obtain their 

 carbon by the absorption of complex organic substances. Green 

 plants can absorb complex carbon compounds, and it has been 

 proved by direct experiment that they can take up these com- 

 plex substances when supplied to their roots. 



