Triassic Roots From The Petrified Forest National Park

DAUGHERTY, LYMAN H. (U. Michigan, Ann Arbor.) Triassic roots from the Petrified Forest National Park. Amer. Jour. Bot. 50 (8): 802-805. Illus. 1963.-Araucariorhiza joae, a new genus and species of Triassic roots, is described. The description includes the root tip with a columella type embryonic region, the secondary roots, the small tertiary roots, and a discussion in regard to the relationship of this form genus. THE FOLLOWING description is based on fossil roots collected by Chief Naturalist Philip F. Van Cleave and on subsequent collections made by Mr. Van Cleave and the writer. The material had eroded from a sandy bentonite of the Chinle formation and a number of specimens were found in a small area at the Itopsidema locality (Daugherty, 1960). This locality is situated approximately 4 mile due north of Agate Bridge in the east central part of Sec. 4, T. 14 N., R. 24 E., within the boundary of the Petrified Forest National Park.2 GENER?AL FEATURES-The fossil roots were discovered within large fragments of a hollow log of Araucarioxylon arizonicum. The log was approximately 35 cm in diameter and the entire hollow center was filled with small roots. The secondary roots are ca. 1-4 mm in diameter and the tertiary roots are ca. 0.4 mm in diameter. Apparently the log had been in a spongy, water-soaked condition before petrifaction occurred, for small branch roots from the central portion of the log had also -penetrated the wood (Fig. 3). Thin sections show perfect preservation in many of the roots, and, judging from the color, the cell walls had retained a considerable amount of carbon after the impregnation of silica formed chalcedony. THE ROOT TIP-A longitudinal section of the tip of a secondary root 630,u in diameter, and with excellent cell structure, proved to have a meristematic region common to many extant gymnosperms (Fig. 1). In many, if not all, of the recent conifers the root cap contains a central columella consisting of transversely dividing cells that also can form daughter cells laterally as described by Esau (1960) for Picea and by De Bary (1877) for Juniperus. The fossil root tip contains a distinct columella, and it is assumed cells of the root were formed in the same manner as those being formed by roots with a columella at the present time. I Received for publication January 26, 1963. The writer is indebted to the Petrified Forest National Park Corporation for expenses incurred on trips to the Petrified Forest, and to the National Science Foundation for financial assistance under grant G-19553, C. A. Arnold project director. 2 The Petrified Forest National Monument became a National Park on December 8, 1962. TERTIARY ROOTS-Small branch roots that were apparently modified for the absorption of water and salts in solution were featured by bulb-like expansions near their tips (Fig. 2, 4). No root caps or root hairs were observed, and epidermal cells of the bulb-like expansions possibly served to carry on absorption. It is not unusual for epidermal cells to function in this manner, for in recent roots having root hairs water absorption may be accomplished by ordinary epidermal cells and is not restricted to the root hairs (Esau, 1960). The stele of the roots is surrounded by an endodermis having Casparian bands and the cortex consists of parenchyma. The stele is diarch, and the cambium was inactive or absent, and, like many small branch roots of extant species, no secondary xylem or phloem was formed. SECONDARY ROOTS-The primary xylem of the secondary roots is essentially like that of the tertiary roots. However, all secondary roots developed root caps and a row of cambial initials on both sides of the metaxylem. When the 2 rows of cambial initials were active, 2 cuneate groups of secondary xylem were formed that appear faiishaped in transverse sections (Fig. 7, 8). Often only 1 row of cambial initials became active, and 1 "fan-shaped" structure of secondary xylem was formed (Fig. 6). When a single "fan" developed, the primary xylem was displaced and forced from the center of the root toward the periphery. The secondary xylem is composed of tracheids with 2 rows of alternating bordered pits on their radial walls (Fig. 5), and, in addition to the typical araucarian pitting, a number of tracheids apparently contain resin plugs. Xylem rays, 2-6 cells high, are uniseriate and consist of procumbent parenchyma cells. In many roots, the phloem is not well preserved, but occasionally it shows excellent preservation (Fig. 6). Prosenchymatous cells ca. 600,u long and 25,u wide are evidently sieve cells although no sieve plates can be observed. Sieve cells of the phloem are approximately square or slightly rectangular in the transverse sections. They are associated with phloem parenchyma and a few isolated sclerotic cells were observed in the periphery of the phloem.