STEMS 



683 



The arrangement of the mestome elements. — In stems the hadrome generally is 

 within the leptome. In dicotyls the mestome strands are arranged in a broken 

 cylinder (6g. S41), which later may become a complete cylinder through cambium 

 activity, while in monocotyls the bundles are scattered, though more abundant out- 

 ward (fig. 550). The most common stem arrangement is collateral, the leptome 

 and hadrome being side by side on the same radius, the leptome outermost (figs. 

 541, 550); in Cucwbita the arrangement is bicollateral, there being a leptome strand 

 inward from the hadrome, as well as outward. In some plants, notably the pterido- 

 phytes, the arrangement of the mestome elements is concentric, the leptome commonly 

 forming a cylinder about the hadrome {hadrocentric arrangement, fig. loio), although 

 there are many cases, as 

 in monocotyl rhizomes, 

 where the leptome is 

 surrounded by hadrome 

 {leptocentric or amphi- 

 vasal arrangement, fig. 

 551). In young roots 

 there are alternating 

 plates of hadrome and 

 leptome in the vascular 

 cylinder {radial arrange- 

 ment, fig. 555); if there 

 are three xylem rays 

 alternating with three 

 phloem rays, the root 

 is called triarch, while 

 such terms as tetrarch, 

 pentarch, hexarch, and 

 polyarch mean respec- 

 tively, four, five, six, and 

 many rays of both xylem 

 and phloem. In the col- 

 lateral bundles of leaves the hadrome is uppermost, even in ferns, in spite of the 

 hadrocentric arrangement in the stems. 



Sheaths encircling the mestome. — In most but not in all cases, there are one or 

 two sheaths or layers of cells surrounding the vascular tract. The inner sheath, 

 regarded as the outermost layer of the vascular region, is known as the pericycle or 

 pericambium (also as the parenchyma sheath or phloem sheath), and commonly is 

 made up of delicate parenchyma cells (fig. 555). Outside the pericycle, and regarded 

 as the innermost layer of the cortex, is the endodermis (also known generally or in 

 special cases as the protective sheath, bundle sheath, starch sheath, or phloeo- 

 terma, fig. 555). The cells are closely packed, and in roots and rhizomes the lateral : 

 and inner walls are suberized (i.e. thickened with suberin, as in cork) and rela- 

 tively impermeable; in aerial stems the layer is less definite and often the cells 

 are rich in starch (whence the name starch sheath). In some roots occasional 

 cells opposite the hadrome plates remain unsuberized for a time and are known 

 as passage cells. 



Fig. ioio. — A partial cross section of a stem of Selagi- 

 nella, showing a hadrocentric (or xylocentric) vascular 

 bundle; note that the thick-walled hadrome (or xylem) 

 cells are surrounded by thin-walled leptome (or phloem) 

 cells; highly magnified. — From Coulter (Part I). 



