164 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



or sand free from humus, the culture media frequently become markedly 

 alkaline, whereas in ordinary soil no such injurious change occurs. The 

 presence of humus is also of importance, inasmuch as it tends to main- 

 tain more equable temperatures, and thus protect the perennial parts 

 buried in the soil. 



A fruitful soil is a conglomeration of mineral detritus and organic 

 remains. Agriculturists distinguish a series of soils, according to the 

 texture, the fineness of division, the relative amounts of coarse and fine 

 particles, and the relation between the organic and inorganic constituents. 



Without entering into the detailed characteristics of different soils, or 

 the methods by which the value of a particular soil is determined, we 



dar 

 Sac 



FIG. 16. From the root-epidermis (piliferous layer) the root-hairs^ and h' arise. The solid soil parti 

 kly shaded \T\ the air spaces (*, y, &c.) are white, the adherent water is indicated by concentric lines. 

 :hs.) 



icles are 

 (After 



may turn to the consideration of a typical humus soil, containing a certain 

 amount of organic material. Such a soil retains a large quantity of water. 

 Air-drying gradually removes a large amount of the water sucked in 

 by the soil, but even during hot dry summer days, a humus soil still 

 retains a certain percentage, although this may be so firmly held as to be 

 unavailable for the plant's use. It is only in an over-saturated soil that 

 the spaces between the soil particles are almost all filled with water, 

 for air is usually present in abundance, especially in the larger interspaces 

 (Fig. 16). 



It is easy to see why the growth of many terrestrial plants is injuriously 

 affected when the soil is over-saturated with water, for the aeration of 

 the soil is then imperfect and the roots are insufficiently supplied with 

 oxygen, while decompositions induced or aided by certain organisms 



