30 THE OAK 



tliis is called the endodermis, and may be regarded as 

 a sheath limiting what belongs to the axis-cylinder (fig. 

 5, a, sh). Inside this endodermis are about two rows 

 of thin- walled cells full of protoplasm, and forming a 

 continuous layer beneath the endodermis, This layer is 

 termed the pericycle (fig. 5, c, per), and it is a very 

 important structure, because its cells give rise, by re- 

 peated divisions, to the lateral rootlets, which then 

 grow out and burst their way through the endodermis, 

 cortex, and piliferous layer, and so reach the soil. It is, 

 of course, necessary to bear in mind that the endodermis 

 and pericycle are concentric cylinders superposed on 

 the axis of the root, as it were, and only appear as rings 

 on the transverse section. 



Inside the pericycle are arranged the vascular 

 bundles, and we shall have to devote a few words of ex- 

 planation to these remarkable and somewhat complex 

 structures. 



The section shows that there are about ten alternat- 

 ing groups of tissue constituting these bundles, and 

 again the reader must bear in mind that each group is 

 the transverse section of a long cord running up and 

 down the root. Of these groups five are much more 

 conspicuous than the other five, because they consist 

 chiefly of more or less polygonal openings with firm, 

 dark contours. These are the xylem vessels of the vas- 

 cular bundles (fig. 5, c, a?), and we must note the following 

 facts about them : In the first place, they are smaller 

 near the pericycle than they are nearer the centre of 



