146 GENERAL STRUCTURE OF ROOT AND STEM 



that the entire account which has been given of the development and 

 disposition of periderm and phelloderm maj' be appHed with special 

 force in the case of the stem. 



The Central Cylinder. — The principal differences between root- 

 structure and stem-structure are found in the developments from the 

 plerom. Although, with the single exception, among important medi- 

 cinal stems, of the male fern, there is but a single stele, in the form 

 of a central cylinder, yet the development of its structure is markedly 

 different from that of the root. Leaving out of consideration exceptions 

 which are unimportant in pharmacognosy, we find that two distinct 

 types of structure characterize respectively the monocotyledons and the 

 dicotyledons and gymnosperms. The form characterizing the latter 

 two will be first considered. 



The Primary Bundles. — Vascular bundles originate in the plerom in 

 the form of a circle, just as in the case of the root, the important differ- 

 ence being that each bundle consists, even in its primary state, of both 

 phloem and xylem, with a cambium between. 



The Open Collateral Bundle. — The typical form is that which in the 

 root constitutes the secondary structure — namely, a bundle consisting 

 of xylem within and phloem without the cambium arc, and this con- 

 stitutes what is known as the Open Collateral Bundle. 



Seco?idary Growth. — Secondary growth here consists in the addition 

 by the cambium to each kind of tissue, and, in almost all cases, the 

 development of new intermediate bundles and new medullary rays, 

 as has been described in the case of the root. The result is that the 

 general plan of structure attained is identical with that already recorded 

 as ultimately attained by the most highly developed woody roots. 

 There are, however, several differences which must be noticed. 



The Medulla or Pith. — The most important is that the primary 

 xylem-bundles do not progress toward and meet one another at the 

 center, so that there is always left there a cylinder of the fundamental 

 tissue, constituting the Medulla or Pith, which is connected through 

 the primary medullary rays with the pericycle, or, after the disappear- 

 ance of that and of the endodermis, with the cortex. 



The whole structure in transverse section may now be roughly com- 

 pared with the wheel of a wagon. The pith corresponds to the hub, 

 the primary medullary rays to the spokes, the spaces between the 

 spokes to the primary wood wedges, the felloe to the bast product, 

 except that the spokes should be seen extending through it, and the 

 tire to the periderm in its various forms of development. 



