68 VASCULAR STRAND 



limited from the cortex by two well-defined layers of cells, an 

 outer, the endodermis (en.), and an inner, the pericycle (p.) (cf. 

 below) . 



In sections stained with aniline chloride a four-rayed group of 

 yellow elements, composing the lignified wood or xylem (Fig. 27), 

 is seen to occupy the greater part of the conducting strand. In 

 the bays between the four arms of the xylem are oval groups of 

 small-celled unstained tissue, the phloem, in which the wide and 

 empty-looking sieve-tubes (s.t.) are plainly distinguished from 

 the narrower companion-cells (ex.) with their dense contents 

 (cf. p. 28). Each phloem -group is separated from the adjacent 

 xylem by one or two layers of parenchyma (ca.). 



The xylem consists chiefly of dead, empty-looking elements, 

 the vessels, of which those at the centre of the conducting strand 

 are the largest, whilst the remainder become progressively smaller 

 in passing outwards along any one of the rays. The end of each 

 xylem-arm, immediately beneath the pericycle, is thus occupied, 

 by a strand of the narrowest vessels (Fig. 27, pt.xy.). In cross- 

 sections, through younger parts of the root, a larger or smaller 

 number of the central* vessels will appear thin-walled and un- 

 lignified, showing that differentiation of the xylem takes place 

 from without inwards. The small peripheral elements are con- 

 sequently spoken of as protoxylem, and the larger, later-formed 

 ones, as metaxylem. 



Longitudinal sections passing through one of the xylem-arms 

 will show that the vessels of the protoxylem are spirally thickened, 

 whilst those of the metaxylem bear bordered pits. The walls 

 between the larger vessels, in the transverse section, exhibit a 

 thin dark line down the middle (the middle lamella), with the 

 thickening layers on either side. These latter are not homo- 

 geneous, however, but appear to consist of short dark lengths 

 alternating with lighter and narrower portions, where the pits 

 are seen in optical section. 



The cells of the endodermis (Fig. 27, en.) are distinctly smaller 

 than the adjacent cortical cells and somewhat flattened. Their 

 radial walls are thicker and look darker than the others, although 

 when sharply focussed in optical section they appear bright owing 

 to their highly refractive character. On treatment of a section 

 with strong sulphuric acid, the membrane swells and dissolves, 



