30 THE OAK 
this is called the endodermis, and may be regarded as 
a sheath limiting what belongs to the axis-cylinder (fig. 
5, C, 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 
erow 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, w), and we must note the following 
facts about them :—TIn the first place, they are smaller 
near the pericycle than they are nearer the centre of 
