ROOTS AND THE SOIL. 1 23 



the shape of a chariot-wheel, or it may be in a single 

 line across the centre ; sometimes it resembles the 

 Isle of Man crest and consists of three rays. A 

 patch of phloem always occupies the interval between 

 the xylem rays. All the other cells inside the 

 endodermis are (i) unspecialised cells which are cap- 

 able of further development. The xylem and phloem 

 do not touch the endodermis, but are separated by a 

 thin layer of these unspecialised cells, called the 

 '' pericycle!' 



The working of such a root is as follows. Water 

 enters the root hairs by " osinosisl' or diffusion through 

 the membranes, and passes in the same way from cell 

 to cell through the cortex, until the cells of the latter 

 are turgid, or swollen with water. The pressure of 

 the water inside these cells may amount to 3 atmo- 

 spheres or 45 lbs. to the square inch. The diffusion 

 inwards or endosmosis of this water depends upon the 

 density of the liquid within the cells, but the proto- 

 plasm exercises control over the process. It can, at 

 will, alter the density within the cell by the secretion or 

 extrusion of substances formed by itself After a 

 certain amount of water accumulates in the cortex, 

 it is forced into the xylem tubes or water-conducting 

 portion of the root. These tubes always tend to be 

 emptied of water, because the water is being evaporated 

 from the leaf above. They form a closed system of 

 water pipes, so far as air is concerned, because the cork 

 between the endodermis cells, above alluded to, prevents 

 the entrance of air from the outside. The spaces 

 between the cortex cells are well aerated ; but air cannot 

 get in through the endodermis elements on account 

 of the cork between them. It must be borne in mind 

 that water readily passes through the cellulose walls 



