142 GENERAL STRUCTURE OF ROOT AND STEM 



The Xylem- or Wood-bundles. — Each of those of one kind (Fig, 422, e) 

 extends gradually toward the center by the successive development 

 there of vessels or cells associated with the vessels of the bundles. Upon 

 meeting there, the bundles, of course, cut off the previously existing 

 central communication between the medullary rays, which are now left 

 as isolated plates or wedges between the bundles, the temporary pith 

 being thus obliterated. These bundles, which meet at the center, are 

 known as the Xylem-bundles or Wood-bundles, and constitute the 

 wood}' portion of the root. 



The Ducts. — The tubes formed as described above are the Ducts. 

 In a few plants which we have to consider, the Gymnosperms, no 

 series of cells lose their end-walls as above described so as to become 

 converted into continuous tubes or ducts, though they connect by 

 perforations. 



The Phloem- or Sieve-bundles and Sieve-tubes. — The other bundles 

 (Fig. 422, /) which have been described as alternating with the xylem-, 

 or wood-bundles, possess as their important element those cells which 

 become connected by perforations in the form of sieves, and are known 

 as the Phloem-bundles. 



Collectively they form what is known as the Sieve-tissue, or Cribrose- 

 tissue, of the plant, and their intercommunicating tubes are the Sieve- 

 tubes. This tissue characterizes the Gymnosperms as well as the 

 Angiosperms. The phloem-bundles do not extend toward the center, as 

 do the xylem-bundles, but stand isolated, each between two medullary 

 rays, which respectively separate it from the xylem-bundle upon either 

 side. 



The Fibers. — In connection with the ducts, or their equivalents 

 in the gymnosperms, and the other tissues of the xylem-bundles, strong 

 fibers develop, the Wood-fibers, while in connection with the sieve-tubes 

 and other tissue of the phfoem-bundles very similar fibers, the Bast- 

 fibers, usually develop. The phloem-bundles, therefore, ordinarily 

 become Bast-bundles. 



Fibro-vascular Bundles. — Vascular bundles in which fibers develop 

 are known as Fibro-vascular bundles. 



Secondary Groivth of the Stele. — The condition now reached by the 

 root constitutes the completed primary structure of its stele. The 

 student should not fail to note that the primary structure refers only 

 to the very smallest roots, and that he need not expect to encounter it 

 in any roots in a condition to be used medicinally. His examination 

 of roots in Pharmacognosy will, therefore, relate to the secondary 



