144 



PHARMACEUTICAL BOTANY 



6. Fibro-vascular bundles of open collateral type which are now arranged 

 into a compact circle, and between which are found primary and often 

 secondary medullary rays. 



From without inward the following tissues make up f. v. bundles. 

 Protophloem J Hard Bast long tenacious bast fibers. 



Secondary Phloem \ Soft Bast phloem cells and sieve tubes. 



Cambium active layer giving rise to secondary phloem on outer and 



secondary xylem or inner face, and adding to depth of med. rays. 

 Secondary xylem wood fibers, pitted vessels, tracheids. 

 Protoxylem spiral tracheae. 

 7. Pith. 



n 



FIG. 71. Portion of cross-section of four-year-old stem of Aristolochia sipho, as 

 shown by the rings of growth in the wood. The letters are the same as in Fig. 68 

 but new tissues have been added by the activity of the cambium; and a cork cam- 

 bium has arisen from the outermost collenchyma cells and given rise to cork. Tho 

 new tissues are; I, cork cambium; k, cork; g, secondary phloem from the cambium, 

 and just outside this is older crushed phloem; n, secondary xylem produced by 

 the cambium; m, secondary medullary ray made by the cambium (notice that this 

 does not extend to the pith). Half of the pith is shown. Notice how it has been 

 crushed almost out of existence. Compare Figs. 68 and 71, tissue for tissue, to 

 find out what changes the primary tissues undergo with age, and to what extent 

 new tissues are added. Photomicrograph X 20. (From Stevens.) 



