126 OUTLINES Of PLANT LIFE. 



derived from the atmosphere or from the soil with which it 

 has come in contact. These substances, whether organic or 

 inorganic, enter the plant, together with the water, through 

 those organs which are adapted to absorption. 



165. 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 mosses the thallus or the leaves are 

 active absorbents. 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 plants the surfaces which are normally 

 in contact with the water are absorbing surfaces. Such 

 plants may be either wholly without a root system, or it may 

 be only sufficiently developed to anchor them in the mud. 

 In land plants the root system is especially adapted to the 

 absorption of water. Only minute quantities of water are 

 absorbed by the leaves and other aerial parts. The root sys- 

 tem of the land plants is developed in contact with the soil. 



EXERCISE XXJ 



To show that water is not absorbed by leaves in quantity adequate to 

 supply evaporation. 



Cut off a vigorous shoot of a plant with abundant foliage; close end of 

 stem with grafting wax ; expose to sunlight until well wilted ; then im- 

 merse it in water. Does the plant recover its turgidity slowly or rapidly ? 



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

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

 ties by which soils are ordinarily distinguished into gravelly, 

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

 is more or less material derived from the offal of plants or 

 animals. When decaying plant offal predominates, the soil 

 is known as vegetable mold or humus, which naturally forms 



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

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



