74 ON THE STRUCTURE OF THE ROOT, 



the upward conduction of water and salts in solution, and one for 

 the passage of prepared food material downwards. 



Between the endodermis and the vascular bundles are one or 

 two layers of cells, known as the pericycle (Fig. 2, pc). This 

 layer is very important, inasmuch as it is the seat of origin of the 

 lateral roots, which all begin to grow in this tissue, and force their 

 way through the cells of the cortex by a process of digestion. 

 The pericycle may also be the seat of changes which result in the 

 formation of a new cortex, at the time when the old one is thrown 

 off, owing to an increase in the size of the root. 



The tissue around the vascular bundles is called parenchyma, 

 or fundamental tissue, and is of the same nature as the cells of the 

 cortex, both having undergone little change since their formation 

 at the apex of the root. The centre of the root is occupied by 

 the pith (Fig. 2, /.), which is also composed of parenchyma, but 

 with somewhat thicker walls. 



If we now examine sections cut in different regions of the root, 

 we shall see that, according as the section is taken nearer or 

 further from the apex, the structure will be less or more complex. 

 In a section quite near the apex, all the cells have practically the 

 same structure ; we find there no differentiation into Xylem, 

 Phloem, Endodermis, etc. Each cell consists of a cell-wall, pro- 

 toplasm, and nucleus. The protoplasm fills up the entire cell and 

 contains no vacuoles (Fig. 5). A little further removed from the 

 apex, we find that the cells have a slightly different structure. 

 Vacuoles appear in the protoplasm, causing it to become restricted 

 to the wall of the cell and to a few strands running out from that 

 which surrounds the nucleus (Fig. 6). At the same time the cells 

 are seen to differ in size, and groups of small cells, arranged regu- 

 larly, represent the future vascular bundles. The further away 

 from the apex, the more does the protoplasm become restricted to 

 the wall of the cell ; and in some cases it disappears entirely. 



We can, in fact, show by a series of sections, longitudinal and 

 transverse, that the formation of new cells takes place at the apex 

 of the root, and that the various tissues of the root are produced 

 by a gradual modification of these. To form the xylem the cells 

 are gradually elongated, the transverse walls between groups of 

 cells placed in rows end to end disappear, and long tubes or 



