NUTRITION. 153 



are spra)'ed, pure water. It always holds in solution sub- 

 stances deri\'ed from the atmosphere or from the soil with 

 which it has come in contact. These substances are either 

 organic or inorganic, and they enter the plant, along with 

 the water, through those organs which are adapted to ab- 

 sorjjtion. 



196. Absorption of water. — In air plants of the simpler 

 sorts, any parts exposed to the moist air or rain can absorb 

 water. In liverworts and mo.sses the thallus or the leaves 

 are acti\e aljsorbents. In the higher plants, such as the 

 aerial orchids, the external cortex of the roots is especially 

 adapted to absorb liquid water, or to condense the water 

 vapor of the atmosphere.* In water jdants the surfaces which 

 are normally in contact with the water are absorbing surfaces. 

 Such plants may be either wholly without a root s)-steni, or it 

 may be only sufficiently developjed to anchor them in the 

 mud. In land plants the root system is especially adapted to 

 tlie absorption of water. Only minute quantities of water 

 are absorbed by the leaves and other aerial parts. The re- 

 vi\al of a wilted plant by spraying seems to be due more 

 largely to checking the loss of water by evaporation than to 

 the slight absorption which may occur. The root system of 

 the land plants is developed in contact ^^-ith the soil. 



197. Soil. — The soil consists primarily of finely divided 

 particles of rock, whose nature and size determine the quali- 

 ties by which soils are ordinarilv tiistinguished into gravelly, 

 sandv, loamy, clayey, etc. Mixed with these rock particles 

 is more or less organic material deri\ed from the offal of 

 l)lants or animals. When decaying plant offal predominates, 

 the soil is known as vegetable mold or humus, which natu- 

 rally forms the upper layer of the soil of forests. To garden 

 or field soils, not naturally rich in organic matter, this is 



* If such condensation really occurs (as is generally alleged), it does 

 not suffice to keep the plants supplied with the required amount of water. 



