114 THE STEM 



of the phloem, viz.: radial rows (one or occasionally two cells 

 in thickness) of living parenchj'ma cells formed from the cam- 

 bium and extending out to the cortex through the phloem ; these 

 radial rows of cells (which of course are actually sheets of tissue) 

 are the phloem rays and are directly continuous across the cam- 

 bium with corresponding rays of the secondary xylem. The 

 phloem rays function both in radial transportation as well as in 

 the storage of food materials. Starch grains are often present 

 in these cells. The sicve-tuhes, which are the important vertical 

 conducting elements of the phloem, appear in transverse section 

 as large somewhat irregular thin-walled cells which appear devoid 

 of protoplasts and are closely joined on their smaller wall-facets 

 with the companion cells. The latter are very small, appear more 

 or less triangular in transverse section and in most cases possess 

 definite nucleated protoplasts. Interspersed among the sieve- 

 tubes are the small isodiametric phloem parenchyma cells which 

 possess a living protoplast. 



Turning now to the wedges of ^'homogeneous tissue," it will 

 be found that these sectors are entirely composed of somewhat 

 "rectangular" parenchyma cells, most of which are provided 

 with a definite protoplast. The characteristic form of these sec- 

 tors is due to a process of dilatation of the multiseriate phloem 

 rays lying between the "banded" sectors of the stem. In other 

 words, certain of the phloem rays (usually those which are two 

 or three cells in width at the cambium) increase continuously in 

 width by the radial and tangential division of the parenchyma 

 cells; the intervening areas of phloem (i.e., those containing the 

 bast fibers and the sieve-tubes) are thus separated from each 

 other and correspond in position to the phloem portions of the 

 original vascular bundles. As the stem increases in age, a similar 

 process of dilatation occurs in the smaller rays of the ])hloem 

 with the result that the "banded" sectors become successively 

 subdivided into smaller groups (for complete details, cf. De Bary 

 pp. 536-537). It should be noted that these dilated phloem rays 

 are directly continuous across the cambium with hi- or triseriate 

 xylem rays. Druses occasionally appear in the parenchyma cells 

 of the dilated phloem rays. 



