PALEOZOIC SEED STUDIES: ON THE GENUS HEXAPTEROSPERMUM

The petrified Paleozoic seed Hexapterospermum is discussed. A new species, H. delevoryii, is described from several seed specimens collected from Middle Pennsylvanian deposits in southern Illinois. The new species is radially symmetrical; approximately 2.5 cm long and 1.3 cm in diam at the widest region. Six primary ribs extend from the integument and give the seed the configuration of a hexagon when sectioned transversely. A thin plate of cells is present at the region of the commissure in each of the ribs. Additional features, not included in the original description of the genus, are described including the occurrence of a pollen-receiving mechanism consisting of a collar-like nucellar extension surrounding a central column. Relationships with other Paleozoic seeds are also discussed. THE GENUS Hexapterospermum was established by Brongniart (1874a, b) for structurally preserved Paleozoic seeds that are hexagonal in transverse section. Seeds of this genus are characterized by the extension of the integumenit into 6 commissured ribs; each rib extends from an angle on the hexagon. Since the original description of the genus, there have been no additional reports nor information regarding either the morphology or detailed structural features of the seed. It is the purpose of this paper to report the occurrence of Hexapterospermum in American coal ball material and to describe in greater detail the structural features of the genus. The description of a new species that follows is based on several specimens collected at the Sahara Coal Company Mine near Carrier Mills, Illinois (Sec. 30; T 9 S; R 4 E, Harrisburg Quadrangle). DESCRIPTION-General morphology-The seeds are ovoid with an acutely tapering apex and flattened base (Fig. 1, 15). In longitudinal section they measure approximately 2.5 cm long and 1.3 cm in diam at the widest part. At the broadest region between apex and chalaza the seed, are hexagonal in transverse section with ribs only slightly developed. Toward the apex the ribs become more pronounced; at the chalaza each of the ribs is extended into a keel-like structure that extends below the base of the seed (Fig. 1). Integument-The integument is constructed of three principal layers. The outer zone is preserved in varying degrees, partial degradation or exfoliation of the outermost cells being common. In specimens where this layer is adequately preserved the tissue is composed of loosely arranged, thin-walled cells that typically show a high degree 1 Received for publication April 5, 1965. This research was carried out during the tenure of a National Science Foundation Postdoctoral Fellowship. 2Present address: Department of Biological Sciences, University of Illinois at Chicago Circle, Chicago, Illinois. of variability in both size and shape (Fig. 9, 14). Cells located toward the periphery of the integument are generally larger (125 A) and possess undulating margins (Fig. 9, 14), while those deeper in the seed coat tend to be smaller and more isodiametric (Fig. 9). None of the specimens showed any evidence of an outer limiting layer or epidermis. In one seed a continuous row of secretory canals was observed near the periphery of the integument (Fig. 9). It is quite probable that additional tissues or integumentary layers were present outside this row of canals in the living state of the seed. The middle layer of the integument is not sharply delimited because there is a gradual transition in cell diameter and wall thickness between it and the outer zone (Fig. 9). In several of the seeds this middle layer is present only as a dark homogeneous zone that forms the "shell" of the seed (Fig. 1, 8). The individual cells are often so tightly compacted that their organization is difficult to determine (Fig. 14). When adequately preserved, however, the individual cells of the layer are narrow in diameter, have thickened walls, and are vertically elongate. Within the integument the long axis of these fiber-like cells parallels the long axis of the seed. Six equally spaced primary ribs extend from the ilitegument at each angle on the hexagon. In transverse section these ribs are most conspicuous toward the apex where they radiate from the integument to give the seed the appearance of a six-armed star (Fig. 4, 5, 16). At the niidlevel the ribs are less pronounced and are represented only as the corners on the equal-valved hexagon (Fig. 6, 17). Basipetal serial sections of the chalaza show the ribs slightly more pronounced (Fig. 18) where they extend below the general configuration of the base to form small flaring keels (Fig. 1, 15). At the region of the commissure of each rib is a thin plate composed of 2-3 rows of thick-walled