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- 

 most continuous layer of 

 phloem, or much more rarely 

 phloem and xylem have re- 



Fio. 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 b undle 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 



