i38 THE STRUCTURE OF ECONOMIC PLANTS 



the transection of a relatively young stem, since they form a definite 

 zone of angular cells lying between the phloem and the cortical 

 region. Their development and characteristics are described in a 

 succeeding section. 



The collateral vascular bundles are separated from one another 

 by medullary rays. The number of primary bundles varies at 

 different stem levels, owing to branching and anastomoses, and to 

 variation from a % spiral to a decussate phyllotaxy. Briosi and 

 Tognini (7) have discussed the course and number of bundles of 

 the stem axis beginning in the hypocotyl. By successive branch- 

 ings, the hypocotyledonary bundles are increased from t to 4, 8, 

 and IX, then reduced by anastomoses to 10. Of these, four com- 

 prise the cotyledonary traces and enter the cotyledons at the cotyle- 

 donary node. In succeeding internodes, the number of primary 

 bundles ranges from it to 16, depending upon the location of the 

 transection examined in relation to a node. Three bundles supply 

 each leaf. 



The primary xylem consists of annular, spiral, reticulate, or 

 pitted vessels. These are differentiated in radial rows, each usu- 

 ally consisting of a single series of progressively larger protoxylem 

 and metaxylem elements that are separated from those of adjacent 

 rows by parenchymatous ray tissue. As secondary thickening 

 proceeds, the development of interfascicular cambium forms a 

 continuous cylinder, and there is an extensive production of xylem 

 fibers, parenchyma, large vessels, and ray parenchyma. Where 

 two vessels lie in direct contact with each other, they are intercon- 

 nected with bordered pits, and simple ones occur in walls separating 

 the vessels from xylem parenchyma and ray cells. The xylem rays 

 are one or two cells in width, consisting of thin-walled parenchym- 

 atous cells whose radial dimensions slightly exceed the tangential 

 ones, and which are approximately twice as high as broad. The 

 elongated, tapered xylem fibers do not attain great length and they 

 are not pitted. A single fiber usually does not exceed 0.5 mm., and 

 the uniformity in length is very striking. 



The phloem consists of sieve tubes, companion cells, parenchyma, 

 fibers, and latex or resin ducts. The sieve tubes have clearly de- 

 fined sieve plates that are either transverse or oblique, and they 

 can be readily distinguished from the ducts by this characteristic. 

 The companion cells are slender and taper to a point where the 

 sieve plate of the adjacent sieve tube is somewhat larger in diameter 



