112 SCIENCE PROGRESS. 



scattered secretory sacs. Owing to the presence of a zone 

 of secondary xylem and phloem, the primary phloem is 

 usually crushed and indistinct, having been separated from 

 the primary xylem by the interpolation of the secondary 

 vascular tissue. One of the most important characteristics 

 of the primary group of xylem is the position of the proto- 

 xylem ; the narrow spiral tracheids occur just internal to 

 the outer edge of each xylem group, and thus occupy an 

 intermediate position, enabling us to distinguish a small 

 group of centrifugally developed scalariform xylem elements 

 and a larger group of centripetally developed pitted tracheae. 



Each of the primary strands of xylem and phloem is a 

 leaf-trace, and in a series of transverse sections it is 

 possible to follow a bundle in its passage from the petiole 

 to the central cylinder of the stem. The course of these 

 foliar bundles is especially interesting ; each extends through 

 about ten internodes. five internodes being traversed by the 

 leaf-trace as it moves outwards from the periphery of the 

 pith to the edge of the secondary wood, and five more in 

 its course through the pericycle and cortical tissues. A 

 trace-bundle on its way through the secondary xylem con- 

 sists of a primary strand and an arc of secondary xylem 

 tracheae, but as it passes further out through the stem 

 tissues the secondary arc disappears and the trace divides 

 into two distinct bundles. Finally in the petiole the twin 

 bundles are frequently reunited. The collateral structure 

 of the leaf-trace is replaced by a concentric arrangement, 

 of the phloem and xylem as the strand reaches the leaf- 

 stalk base. 



The secondary vascular tissues met with in practically 

 all stems of Lyginodendron consist of radiating rows of 

 pitted tracheae, separated by numerous large medullary 

 rays, and of narrow phloem cells associated with sieve-tube 

 like elements. Secretory sacs are fairly common in both 

 xylem and phloem rays. In several specimens the charac- 

 teristic tabular cells of the cambium have been preserved, 

 and there can be no doubt as to the perfectly normal 

 manner of development of the secondary wood and bast. 

 External to the phloem, and constituting the limits of the 



