STRUCTURES DEVELOPED FROM THE PLEROM 141 



No Fibro-vascular Tissue Developed from P'eriblem. — No tissue 

 developed directly or indirectly from the periblem is in the form of 

 distinct and regular bundles of vessels, though irregular and isolated 

 or anastomosing tubes are frequently developed by it. 



Structures Developed from the Plerom. — The Stele. — The essential 

 characteristic of the body developed from the plerom of the root is 

 that it is invested by the endodermis and is free from any other endo- 

 dermal development in any part. It, therefore, constitutes a Stele 

 (all inside of c), which in the root is always in the form of a Central 

 Cylinder. ' 



Differentiation in the Cells of the Stele. — The plerom exhibits at first 

 only slight differences in the appearance of its cells (Fig. 420, a), and 

 a transverse section of it viewed with the microscope might be figura- 

 tively compared to looking down upon a honeycomb built in a cylin- 

 drical tin box, the latter representing the endodermis, and in longitudinal 

 section to a longitudinal section through the same. This constitutes 

 the Ground-tissue of the Stele. Farther away from the tip, however, 

 it would be found that groups of its cells (Fig. 421, e and/) had elon- 

 gated in a longitudinal direction, and these, to continue our illustration, 

 might be compared to bundles of pencils or quills set in the honeycomb. 

 Mingled among the elongated cells of the bundle, however, are many 

 which have not elongated. 



Medullary Rays. — These bundles would be arranged in a circle 

 separated from one another by more or less of the honeycomb tissue, 

 these separating portions corresponding to the Medullary Rays of the 

 Stele (g). 



The Pericycle. — From the endodermis they would be separated by 

 one or more continuous circles of the honeycomb cells, corresponding 

 to the Pericycle or " Pericambium" (/;)• For a time there would also 

 be left a central portion (i), consisting of unchanged cells, forming a 

 temporary Medulla or pith. 



The Vessels. — The elongated cells, which constitute the important 

 elements of the bundles, are joined end to end with other similar ones 

 still farther up in the older part of the structure. At first the end walls 

 of these abutting cells separate their cavities from one another, but 

 later these disappear in those of some bundles, becoming perforated in 

 those of others, so that the cavities become more or less continuous, 

 forming the Vessels, extending throughout the root and into and through 

 the stem above. The bundles thus formed are thus of two kinds, alter- 

 nating in the circle. 



