VEGETATIVE GROWTH OF EORHIZA-ARNOLDII ROBISON AND PERSON FROM THE MIDDLE EOCENE PRINCETON CHERT LOCALITY OF BRITISH-COLUMBIA

Anatomical studies of several thousand specimens of Eorhiza arnoldii R obison and Person rhizomes and their attached organs have added signif icantly to our knowledge of these Middle Eocene, semiaquatic dicotyled onous plants from the Princeton chert. Anatomical structure of Eorhiza was studied through serial sections on cellulose acetate peels. In or der to establish the growth habit for these plants, individual axes we re mapped as to their three-dimensional positions in the chert matrix. Plants grew from an extensive sympodial rhizome system similar to man y living monocots and exhibited subopposite branching. Branches gave r ise to rhizome sympodia or to branches bearing small scale-leaves. The architecture of Eorhiza conforms to the Tomlinson Model proposed by H alle, Oldeman, and Tomlinson. Leaves are ensiform, equitant, unifacial (isobilateral), and monocot-like with a large central lacuna. Rhizome s show typical dicot stelar anatomy. The presence of an aerenchymatous cortex and the plants' association with freshwater animal remains and representatives of known aquatic plant families, such as Nymphaeaceae , support an aquatic habitat for Eorhiza. The presence of rhizomes tha t produce roots bearing secondary and tertiary roots indicates in situ preservation of extensive rhizome systems at least 41 cm in length. A reconstruction of the vegetative body of Eorhiza is presented and a h ypothesis offered concerning the attachment of reproductive structures to these plants.