RESEARCH ON XYLEM AND PHLOEM 99 



curiosity about this phase. If one merely uses a razor blade for sectioning and 

 a lOX hand lens for observing, he can learn a great deal about the configura- 

 tion of xylem and phloem in the plant. By using cross sections (taken at right 

 angles to the long axis — they are thus similar to a normal slice of bread) 

 and choosing young stems of many dicotyledons for research materials, it is 

 apparent that the patterns of vascular tissues are remarkably constant 

 throughout thousands of species, even though the details may differ. Many 

 other dicotyledons could be chosen, on the other hand, in which the patterns 

 are different ; they may be, for example, more similar to those generally found 

 in thousands of monocotyledons. If attention is turned to the ferns, even 

 greater variation could be found. But if one similarly examined cross sections 

 of vigorously growing young roots (a few inches or so back of the tip), he 

 would find quite a different pattern, one which has some characteristic fea- 

 tures that make it easy to distinguish roots from practically all stems just 

 on the basis of the arrangement of xylem and phloem. 



Late in the nineteenth century anatomists began pondering the evolutionary 

 significance of these patterns, and from their efforts emerged the concept of 

 the stele. The stele was considered as the central cylinder of the stem or 

 root and was composed of the vascular tissues and others closely associated 



slightly oblique end wall (fig. 4) or in transverse end wall (fig. 5). — Fig. 6. Bordered 

 pit in surface view. — Fig. 7. Bordered pit pair sectioned through center. Borders of 

 secondary wall of two cells concerned appear like opposed sets of two spread 

 fingers ; continuous pit membrane across the cavity actually separates the two cells — 

 in a perforation the common pit membrane disappears. — Fig. 8. Section through a 

 simple pit pair; no borders of secondary wall. — Fig. 9. Diagram of part of a cross 

 section of stem four years old, showing pith, protoxylem {PX), and metaxylem 

 {MX), which compose primary xylem, early secondary xylem {ESX), later second- 

 ary xylem {LSX), wood rays {WR), vascular cambium (FC), secondary phloem 

 (SFH) with vertically oriented parts stippled, phloem ray (PR). — Fig. 10-11. Parts 

 of two xylem conducting elements with annual thickenings (fig. 10) and spiral 

 thickening (fig. 11). — Fig. 12-14. Parts of xylem conducting elements with scalari- 

 form (fig. 12), opposite (fig. 13) and alternate (fig. 14) pitting. Borders of pits not 

 shown in fig. 12 and 13. — Fig. 15. Outline of shortened sieve cell; sieve areas not 

 shown but would be of equal specialization and scattered along the entire wall. — 

 Fig. 16. Sieve-tube member with oblique sieve plates [five oval sieve areas (SA) 

 in upper and four in lower]. Dots merely represent sieve area in contrast to re- 

 maining part of end wall. — Fig. 17. Assemblage of cells derived from one phloem 

 initial: sieve-tube members (STM) with sieve plates on end walls, strand of 

 parenchyma cells (P) with nuclei, stippled companion cells (CC). — Fig. 18. Sieve- 

 tube member in section, showing sieve plates (SP) on transverse end walls and 

 two companion cells (CC). — Fig. 19. Diagram of part of sieve plate in section; 

 connecting strands (CS) in black, wall (W) cross-hatched, callose cylinders (CAC) 

 clear. — Fig. 20. Part of sieve plate in surface view; connecting strands (CS) in 

 black, callose cylinders (CAC) clear, wall (W) in black. 



