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. 
