56 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
taking place first in a few cells and finally including all 
the procambium in the so-called closed bundles or 
leaving a sheet of unchanged meristem between the 
xylem and phloem in the so-called open bundles. 
81. Classifying them by the relative positions of the 
xylem and phloem parts of the bundle, we may dis- 
tinguish three main types of vascular bundles, radial, 
concentric, and collateral. In the radial type, the 
xylem is present in two to many radially situated, more 
or less flattened strands, which may or may not reach 
the center. Alternating with these are the masses of | 
phloem. In the concentric type, the xylem is central 
and is surrounded by an al- 
€3 | : eat: Shut Py 
Ji _ (oe > (255 most continuous layer of 
‘al Ved Sy ey) phloem, or much more rarely 
phloem and xylem have re- 
Fic. 23.—Plans of radial, concentric, Verse positions. In the col- 
and collateral vascular bundles. 
lateral type, the xylem occu- 
pies one side of the bundle (usually that toward the 
center of the stem), and the phloem the other side 
(usually the centrifugal side). 
82. The radial vascular bundle is typical of roots. 
It occupies that part that was marked off as plerome at 
the growing point. Bounding it is a layer of rather thick- 
walled cells, often with suberized or cutinized walls, the 
endodermis (or bundle sheath). ‘This is actually the inner 
layer of the cortex, and is not really a part of the bundle 
itself. Within this is a delicate layer of thin-walled cells, 
the pericycle (or pericambium). Bordering on this, 
or in some families of plants interrupting it, and therefore 
touching the endodermis, are the xylem strands. ‘These 
are made up of tracheary tissue. The elements vary in 
size, the smallest (those first differentiated from the pro- 
cambium) being those next to the pericycle, those lying 
al dee 
