220 BOTANY AND PHAkxMACOGNOSY. 



atmosphere and partly from the soil. Those elements derived 

 from the air are either themselves gases or exist in combination 

 in the form of gas, and include oxygen, nitrogen in exceptional 

 cases (p. 99 j, and carbon dioxide, the source of the carbon 

 entering into the carbon compounds formed by plants. 



The elements obtained by plants from the soil exist in com- 

 bination with other elements and must be in the form of solution 

 to be absorbed. The soil consists largely of mineral substances, 

 together with certain organic products (humus). The water held 

 in the soil not only acts as a medium for carrying the soluble 

 constituents in the soil to the plant, but is itself an important food 

 product, being the source of the hydrogen used by plants, as also 

 of assimilable oxygen. Among the mineral constituents of the 

 soil that are useful to plants are ammonium salts and nitrates, 

 sulphates, phosphates, chlorides, silicates and carbonates. When 

 plants are collected and subjected to a temperature of about iio I 

 C. the water is driven off, and then if heat sufficient to incinerate 

 the material be applied the organic matter is driven off in the 

 form of gases, leaving the mineral constituents in the form of ash, 

 as calcium, magnesium, iron, potassium, sodium and a few other 

 elements. 



Root Absorption. Notwithstanding the various agents 

 which are at work tending to break down and dissolve the sub- 

 stances contained in the soil, as soil bacteria, the liquids given to 

 the soil by the roots of the plants themselves, the presence of the 

 so-called humic acids, and the action of water and air, it has been 

 shown that the soil water is an exceedingly weak solution. This 

 is largely due to the peculiar absorptive and fixing power of the 

 soil itself. 



The dilution of the aqueous solution of the soil constituents 

 is a matter of very great significance, for upon this depends its 

 absorption by the root hairs. While other parts of roots have cer- 

 tain absorptive powers, the root hairs have been defined as the 

 organs of absorption of the plant. They are very delicate in 

 structure and contain protoplasm. Their absorbent function de- 

 pends upon the principle that w^hen a membrane (animal or 

 vegetable) is interposed between two liquids of unequal density, 

 the less dense liquid will pass through the membrane and mix 



