THE ROOT AND ITS FUNCTIONS 



53 



moment its concentration becomes equal, within and without 

 the cell, the movement of this particular substance ceases, even 

 though others are passing rapidly through the membranes. 



The Absorption of Water and Salts. — This activity of the cell 

 as an osmotic sj'stcm evidently controls its most important 

 functions. Let us first consider the role played by osmosis in 

 that process which is the immediate subject of this chapter, the 



Fig. 30. — Movement of water and dissolved substances into the root. Dia- 

 gram showing the entrance of water and dissolved salts into two root-hairs and 

 their passage thence through the cortical cells of the root. The cytoplasmic 

 membranes are readily permeable to water and salts but prevent the passage of 

 sugar dissolved in the cell-sap. 



absorption of water and nutrient materials from the soil (Fig. 

 30). Each root-hair, as we have seen, is merely a projection 

 from one of the epidermal cells of the root. The cytoplasm 

 and sap-cavity of the cell extend into the root-hair, the whole 

 of which is thus lined by a thin cytoplasmic layer, Avith its 

 membranes (Fig. 25). The root-hair penetrates the soil and 

 comes intimately in contact with the soil-particles, to the sur- 

 face of each of which a thin water-film normally adheres. In this 

 water are dissolved a great variety of substances, but the total 

 concentration of the soil solution is normally less than that in the 



