THE STRUCTURE OF THE ROOT 33 



these separate spaces become united, so that intercellular 

 passages run among the cells of every region, being of 

 different dimensions in different areas. As we shall see 

 later these passages become open to the exterior in the 

 upper portion of the plant and so enable air to enter and 

 circulate in the interior of the tissues. 



enpeph x p x px 



PIG. 12. Section of central part of root. In the outer region the cells 

 are separated in places by the intercellular spaces. en, endo- 

 dermis; pe, pericycle; ph, phloem strand; p, pith; x, xylem 

 strand; px, protoxylem. Xioo. (After Kny.) 



A longitudinal section of the root taken at this age 

 will show that beside the longitudinal areas or regions 

 already remarked the internal tissue is beginning to be 

 differentiated in another direction. The section of the 

 root is almost conical, but the apex of the cone can be 

 divided into three layers, each of which is continued 

 backwards alone: the axis. At the apex each layer can 



c 



