1920] LANGDON—DIOON SPINULOSUM 123 



the sheathing monocotyl leaves, save that the marginal vein of the 

 typically sheathing monocotyl leaf is connected with a large number 

 of bundles which come off around the entire periphery of the stem. 



Summary 



e 



1. The medullary rays of Dioon spinulosum are of three distinct 

 types: uniseriate rays, a single layer of cells wide and several cells 

 deep; multiseriate rays, two to several cells in width and of vari- 

 able longitudinal extent; and broad foliar rays or leaf gaps, which, 

 with their included leaf traces, are such a constant feature of this 

 wood. 



2. The fibrovascular elements constituting the leaf traces in 

 the foliar rays and connecting these traces with the secondary 

 wood are peculiar, irregular scalariform tracheids which in the 

 course of their development curve gradually downward through 

 the ray, until they become inserted between the perpendicular 

 fibrous elements of the main stele. 



3. The regular pitted elements of the secondary xylem are also 

 often diverted to one side into a direction parallel to that of the 

 trace. 



4. Both the scalariform and the pitted elements constituting 

 these traces, in their peculiar manner of enlargement and elonga- 

 tion, furnish excellent illustrations of gliding growth. 



5. For each leaf or scale leaf 7-9 strands (the number varying 

 with the size of the leaf base) separate from the vascular cylinder. 

 The two inner ones, arising from the proximal side of the central 

 cylinder, pursue a more or less direct vertical course into the 

 ventral part of the petiole; the rest of the traces, leaving the stem 

 cylinder at different points, pass obliquely upward into the cortex 

 and the sheathing base of the leaf, where an anastomosis of traces 

 takes place, resulting in the two characteristic girdles. 



6. The two direct strands entering the ventral or abaxial part 

 of the leaf may also unite with the two dorsal girdling strands at 

 the base of the petiole, so that the whole system is reducible in the 

 older seedlings and adult stem to two main horizontal strands with 

 their associated lateral traces. 





