296 THE ROOT 



that the thickening no longer supports the wall and 

 the tracheids are crushed (Fig. 45, F). , 



A transverse section across the elongating region | 

 shows the protoxylem elements already lignified, but 

 the cells which will develop into metaxylem vessels 

 still thin walled and living. In the root^hair region 

 the walls of these gradually thicken and lignify, and 

 various stages in their development can be traced. 



Absorptive Function oJ the Root. — Owing to the 

 existence of substances such as sugar, which attract 

 water, in the cell sap of the root hair, water is drawn 

 in from the soil outside. As the solution within the 

 vacuole becomes more dilute, the absorptive power of 

 the root-hair cell decreases, and when the dilution 

 has passed a certain point the absorptive power of the 

 adjoining cortical cells, whose cell sap will then be more 

 concentrated than that of the root-hair cell, will cause 

 water to be drawn into them from the root-hair cell. 

 In the same way water will be drawn across the cortex 

 from cell to cell through the endodermis and pericycle 

 to the xylem tracheids and vessels. The mechanism 

 of the passage of water from the living cells of the 

 cylinder into the xylem tracheids and vessels is not 

 fully understood, but it seems probable that osmotically 

 active substances exist in considerable quantities in 

 these also. When they lose their protoplasm after 

 differentiation a considerable amount of organic 

 substance must be left in their cavities. 



It must be clearly understood that the passage of 

 solutes, for instance the mineral salts — nitrates, sulphates 

 and phosphates — absorbed from the soil and ultimately 

 passing into the xylem, do not necessarily travel across 

 the cortex at the same rate as the water. Their passage 

 depends on the state of equilibrium between the different 



