92 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



(2) The cambium, of long narrow cells arranged in regular 

 rows and having abundant contents and very thin walls. 



(3) The xylem vessels, embedded in parenchyma. The 

 large outer pitted vessels appear to consist of a row of 

 empty oblong cells bearing on their walls a network of 

 thickening with thin meshes (pits) ; but closer examination 

 and focussing shows that the apparent cross- walls are merely 

 ring-like projections representing the remains of the origin- 

 ally complete transverse walls that have been almost entirely 

 absorbed in the formation of the vessel from a row of cells 

 end to end ; of the inner (protoxylem) vessels some have 

 spiral and others annular thickenings on the inner surface of 

 the walls. 



(4) The inner phloem, like the outer in structure. 



(c) Now examine the sieve-tubes (as seen in longitu- 

 dinal section of stem) in greater detail. In the older tubes, 

 especially in material cut in late summer or autumn, each 

 sieve-plate* is covered with a mass of callus, which stains 

 yellow-brown with iodine but is readily distinguished from 

 the proteid contents of the tube. Irrigate with potash ; 

 the callus swells and becomes transparent, so that the 

 cellulose portion of the plate becomes conspicuous and 

 shows the pores, the plate appearing in optical section (on 

 focussing into it) like a string of beads the constrictions 

 corresponding to the pores. Other sections showing callus 

 on the sieve-plates should be treated with callus reagent 

 (see Appendix) which stains it brown, and with corallin 

 (see Appendix) which stains it pink. 



(d) That the proteid contents (which often collect in a 

 clump in contact with the plate, especially on the upper 

 side) are continuous from segment to segment of the tube 

 through the pores may be shown by either of the following 

 methods both should be tried. (1) Treat a section with 

 iodine, wipe it with blotting-paper, mount it in a small 

 drop of strong sulphuric acid, and very carefully cover and 

 examine it. The acid causes the cellulose and the callus to 

 swell up and the protoplasm to contract, so that the proteid 

 contents appear as strands with here and there a thicken- 

 ing corresponding to the position of a sieve-tube each 

 such thickening will show the fine proteid strings which 

 pass through the pores of the plate. (2) Add some dry 



