of Goal-Measure Plants. 159 



the lighter primary xylem (x 1 ) stands out clearly in contrast to the 

 darker secondary wood (x 2 ). The leaf-trace (It) of fig. 2, PI. v. is 

 here seen in longitudinal view ; this section demonstrates very 

 clearly the gradual merging of the leaf-trace elements into those 

 of the primary stele of the stem as they are traced downwards ; at 

 the top of the section — where the letters It are printed — the strand 

 of dark lines denotes a compact and well-defined group of tra- 

 cheids, but these are seen to become more and more indistinct 

 and to spread out laterally when traced downwards ; the leaf- trace, 

 in fact, loses its individuality and becomes indistinguishable from 

 the main mass of the xylem. Having sketched the most obvious 

 features of the stem, or at least of the small portion available for 

 examination, it remains to consider in detail the several tissues. 



1. Secondary xylem. 



Fig. 4, PI. v. illustrates the structure of the secondary wood 

 as seen in transverse section ; it is characterised by the pre- 

 sence of numerous medullary rays of parenchymatous elements, 

 1 — 5 cells in breadth, and is composed of bands of tracheids, 1 — 8 

 rows in width. There are no regular concentric rows of narrower 

 elements marking rings of growth, but occasionally groups or dis- 

 continuous lines of narrower elements afford indications of local 

 irregularities in the production of the secondary elements. Towards 

 its inner margin the secondary wood becomes less dense, the bands 

 of tracheids are reduced in width, the medullary rays gradually 

 widen, and their parenchymatous elements become tangentially 

 elongated towards the periphery of the primary stele (PL v. fig. 3, x 2 , 

 PL vi., PL vii. fig. 8). The medullary-ray cells in transverse 

 section have the form of narrow radially elongated elements with 

 transverse or oblique walls, and some of them contain a dark 

 substance, suggestive of a product of secretion. In tangential 

 section the secondary xylem presents the appearance of a reti- 

 culum of tracheids with the spaces filled in with comparatively 

 broad and long masses of medullary-ray parenchyma ; some of the 

 medullary rays have a length of 5 mm. as viewed in a tangential 

 section of the stem ; their slightly elongated polygonal elements 

 are separated by small intercellular spaces. The rays vary in 

 breadth from one to several cells as seen in tangential section 

 (sections I. 4, I. 5). The tangential walls of the tracheids are often 

 marked by a fine oblique striation, and occasionally two intersecting 

 series of delicate spiral lines may be detected ; this striation is 

 probably an original structure emphasized or etched out by ferment- 

 action during decay. A radial section through the secondary 

 xylem illustrates more clearly the abundance of parenchymatous 

 tissue ; deep plates of medullary-ray cells, with an occasional 



12—2 



