CELLS AND TISSUES 31 



In fleshy storage organs, the proportion of xylem parenchyma 

 may be very large as in the root and hypocotyl of Raphanus and 

 Beta; and, in such cases, the cells may remain active and greatly 

 increase in number. In woody axes, xylem parenchyma is limited 

 or may be totally lacking except in the xylem rays. Where second- 

 ary thickening occurs, parenchymatous cells cut off by the cambium 

 constitute the xylem rays, which may be one to several cell layers in 

 width. The ray appears in a radial longisection as a sheet of tissue, 

 and its height may vary from few to many cells. In herbaceous 

 plants, the xylem ray cells are parenchymatous; but, in woody 

 ones, they may become sclerenchymatous and lignified, eventually 

 forming the most resistant portions of the wood. Unlike the 

 medullary rays, the xylem rays may have no direct contact with the 

 pith, but the latter are in intimate contact with the vertically 

 oriented elements of the stele, functioning in transverse conduction, 

 and serving as a storage region, especially in semi-woody biennials 

 and perennials. 



The Phloem. — Like the xylem, the phloem is a complex vas- 

 cular tissue of the stele. It may include sieve tubes, companion cells, 

 fibers, parenchyma, and secretory cells; but, frequently, one or more of 

 these elements are lacking. 



Sieve Tubes. — The sieve tube is the principal conductive unit of 

 the phloem; but parenchyma is important in this respect, especially 

 in the protophloem, in which sieve tubes may not be differentiated 

 and the elements consist of slender, elongated, parenchymatous 

 cells. Structurally, the sieve tube is a vertical series of elongated, 

 thin-walled cells which are interconnected by perforations in their 

 walls occurring in areas known as sieve plates. The end walls may 

 be transverse, oblique, tapered, or beveled; and the sieve plates 

 occur in them, or the lateral walls, or both. Although many 

 angiosperms have lateral plates, there is commonly a single trans- 

 verse one which is perforated by numerous pores through which the 

 protoplasmic strands extend as continuations of the cytoplasm of 

 the sieve tube segments. It has no nucleus at maturity, but may 

 contain minute starch grains and plastids. Slime bodies have been 

 observed in the cytoplasm of the sieve tubes of many plants, in- 

 cluding Beta, Cucurbita, and Solanum; and from the proteinaceous 

 contents of the cytoplasm and cell sap, slime plugs may form which 

 penetrate the pores of the sieve plate. 



Hill (x3) and others have described the protoplasmic strand as 



