yS SIEVE-TUBE ELEMENTS 



primary walls. In various members of the Abietineae, however, 

 Abbe and Crafts (1939) reported true secondary walls in the 

 sieve-elements of the secondary phloem. According to Esau's 

 review, the nature of the pits or protoplasmic connections of 

 sieve-tube elements with companion cells and phloem parenchyma 

 cells is not yet definitely established. In some cases at least, the 

 wall between the sieve-tube element and its companion cell is 

 penetrated by numerous scattered plasmodesmata. 



II. Material for the Study of Sieve-Tube Elements. — 



1. The phloem of Cucurbita. Obtain a thin transverse sec- 

 tion of the stem of pumpkin and examine its structure under low 

 magnification. In progressing from the edge of the section 

 toM'ards the center the following tissues may be observed, viz. : 

 (1) a typical uniseriate epidermis, certain cells of which have 

 developed into hairs; (2) a rather narrow eortex, consisting of 

 an outer, discontinuous zone of angular eollenchyma followed by 

 a region of parenchyma the innermost layer of which may contain 

 abundant starch grains and appear as a starch-sheath if the sec- 

 tion is stained in iodine; (3) the stele, the outer boundary of 

 which is clearly indicated by a continuous cylinder of thick- 

 walled pericyclic fibers. Internal to the fibers occurs a' broad 

 parenchymatous zone in tlie innei- portion of which are found 

 two "rings" of vascular bundles. Each bundle consists of a 

 median strand of xylem (characterized b}^ its large vessels) 

 flanked on both sides by a strand of phloem. A bundle of this 

 type is designated as a bicoUateral vascular bundle. Clear evi- 

 dence of a cambial zone may be seen between the xylem and each 

 of the phloem strands, especially in the larger inner bundles. 

 The center of the stem is occupied by an irregular cavity which 

 was produced by the collapse and disintegration of the pith. To 

 investigate the styurture of th( phloem, remove the cover-gla'ss 

 mid mount the section in a .1% aqueous solution of aniline blue.^ 

 This dye "will stain the callus depositions on any of the sieve- 

 plates which may be present in the section. A careful study, 

 vnder high magnification, of the phloem of the various bi-col- 

 lateral bundles will usually reveal a number of large sieve-plates. 

 These structures in Cucurbita occupy virtually the entire end- 



1 Cf. Appendix, p. 142. 



