VASCULAR BUNDLE 79 



as in the case of the root, is a purely topographical one, being 

 applied to the region situated between the conducting strands 

 and the starch-sheath or a similar continuous layer of cells. 



In the individual bundles the phloem can be distinguished 

 by the shining appearance of the cell-walls, which resemble those 

 of the young fibres, as well as by the unequal size of its com- 

 ponent elements (Fig. 35). The large, empty-looking sieve-tubes, 

 exhibiting an occasional sieve-plate (s.p.) 1 and associated with 

 small companion cells (c.c.), which have dense contents, are inter- 

 mingled with ordinary parenchyma-cells (phloem-parenchyma, 

 ph. p.) . Two or three layers of flattened thin-walled cells, situated 

 between the phloem and the xylem, constitute the cambium 

 (cf. Fig. 57, C, p. 118), a meristematic region which becomes 

 active in the older stem and forms additional vascular tissue 

 (cf. Chapter X). 



The wood consists of radial files of vessels separated by rows 

 of smaller parenchymatous cells (the wood-parenchyma, Fig. 35). 

 The smallest vessels (Pt.xy.), which represent the protoxylem 

 (cf. p. 68), are situated nearest the pith, while the larger 

 metaxylem-elements (M.xy.) are towards the outside, a further 

 point of contrast to the root. The wood-parenchyma cells have 

 more or less thickened walls, which are lignified like those of the 

 vessels, but they nevertheless retain their living contents. Small 

 cells resembling the wood-parenchyma, but not lignified, form 

 a sheath (the medullary sheath, Fig. 34) at the inner edge of each 

 bundle. 



A longitudinal section of a Sunflower-stem passing radially 

 (i.e. parallel to a radius) through one of the bundles will show 

 the same succession of tissues and enable us to complete our 

 picture of the various elements (Fig. 36) . The short and some- 

 what bulging epidermal cells are succeeded by those of the 

 collenchyma, which are many times longer than broad, and 

 have strongly thickened longitudinal walls (cf. Fig. 16, A, of 

 Dead-nettle). The thin-walled rectangular cells of the cortex 

 (Fig. 36, Co.) are much shorter. Next come the starch-sheath 

 (sh.), recognisable by its large starch-grains, and the typical fibres 

 of the pericycle (/.). The detailed structure of phloem and cam- 

 bium (c.) will be studied in another stem, but in the former the 

 1 The sieve-plates are not readily recognised in the Sunflower. 



