14 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA COLLEGE, 
abruptly or gradually through a layer of somewhat larger parenchymatous cells into the 
starch-ring. ‘This latter element is always conspicuous, and is usually made up of large 
oblong or parallelopipedal cells lying in one or several rows. The starch-ring lies 
against the third cylinder and is generally continuous, being interrupted in some cases, 
however, by the meeting of the hypoderma and stereome. The periblematic cylinder 
undergoes no secondary differentiation, but remains in its primary condition throughout, 
except in subgenera Avicularia and Duravia, where fascicles of stereid cells occur; in 
the former they lie next to the epidermis, while in the latter their position is near 
the starch-ring. 
THE PLEROMATIC CYLINDER. In this four main elements are represented, namely, 
the stereome, the phloem, the xylem and the pith. Next to the starch-ring comes 
either a strong sheath or fascicles of stereid cells. This tissue in the majority of cases 
forms a stereome cylinder. Sometimes the walls of these cells are so thick that they 
almost obliterate the cavities, while in other cases they are comparatively thin. In case 
no cylinder is found this tissue is represented by fascicles at the outer side of and radi- 
ally with the collateral bundles. 
The phloem occurs mostly as fascicles between the stereome cylinder and the xylem, 
appearing in cross-section as areas ranging from oblong to almost linear, usually widely 
separated but rarely nearly continuous but only slightly disconnected between the bun- 
dles by the sclerotic or starch-ring elements. 
The xylem is developed according to the age and character of the individual. It is 
composed of tracheae, wood-parenchyma and intermediate tissue, the two latter being 
most plentiful around the larger elements and at the inner ends of the bundles, where 
they pass into the pith and in many cases appear as much like cellular tissue as does 
the pith. 
The pith is made up of typical parenchymatous cells, which are polyhedral and 
either isodiametric or more or less elongated, with the exception of one or two instances 
in sections Persicaria and Tovara, in which the cells are broader than high. 
PALAEONTOLOGY. 
Several species of Polygona and polygonaceous plants have been known to occur in 
the Tertiary strata of the Old World. These have been described from perfect and im- 
perfect leaf specimens and fruits. Some of the species are well marked and character- 
istic, and can be referred to their respective subgenera, while others described from small 
fragments of leaves are rather doubtful. 
