1922] BENSON—HETEROTHECA GRIEVII 127 
The vertical plates in the periphery often have their cells thick- 
ened with layers of cell wall substance which almost obliterate 
the lumen of the cell; other plates are small, but most show the 
hexagonal form of cell indicated in text figs. 3 and 4. Abutting 
on these are tubular cells which 
are disposed mainly in the 
horizontal or transverse plane 
(text fig. 3). In the fossil 
these are generally swollen, and 
owing to their form and pale 
color have somewhat the ap- 
pearance of macaroni. Some 
fibers accompany the delicate 
branches of the vascular 
bundles, and are continuous 
with a parenchymatous hood- 
like envelope to the nerve end- 
ing. Many, perhaps more in 
some specimens than in others, 
show a spiral line of thickening 
which resists imbibition and 
consequent swelling. 
The six to eight mesarch 
strands of primary xylem which 
travel mainly in the peripheral Fic. 7.—Camera sketch of specimen of 
region give off the delicate Heterotheca in CN.288.2; no carbonaceous 
centroscopically directed xylem matter is retained in center, and therefore 
Sivands whidk ond in tie ified elements of ground tissue appear 
’ exceptionally clearly; five nerve endings, 
aqueous storage tissue. The many transfusion elements, and two vascu- 
contrast between the short _ lar bundles can be distinguished in original; 
barrel-shape d tracheids of the ee RS of vertical plates (vsp) charac- 
i teristic; striated elements seen near apex as 
nerve ending and the slender jn many cases (cf. text fig. 3). 
carriers of the water is quite 
reminiscent of highly organized water storage organs in recent 
plants. The majority of the nerve endings in the synangium were 
directed to its central region. Transfusion elements of varying 
degrees of specialization are exceedingly abundant in some speci- 
