PLANT ORGANS AND ORGANISMS 
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. 
Pith. 
Fig. 44. — Portion of cross-section of four-year-old stem of Arislolochia sipho, as 
shown by the rings of growth in the wood. The lettters are the same as in Fig. 41 
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. The 
new tissues are; l, 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. 41 and 44, 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.) 
