DESCRIPTION OF THE PHYLUM TRIMEROPHYTOPHYTA (BANKS 1975)

EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> STREPTOBIONTA> EMBRYOPHYTA> TRACHEOPHYTA> TRIMEROPHYTOPHYTA

Trimerophytophyta (tri-me-ro-fi-TA-fa-ta) is formed from three Greek roots that mean three (tria -τρία); parts (meros -μέρος); plant (phyto -φυτό).

INTRODUCTION TO THE TRIMEROPHYTOPHYTA The Trimerophytophyta resembled the rhyniophytes, so much so that Bierhorst (1971) considered them a family of that group. They had terminal homosporous (text with tooltip) Homosporous (adj) plants produce one type of spore. sporangia with longitudinal slits that arose from dichotomizing lateral branches (text with tooltip) Dichotomous branching is the simple pattern of branching in which each node produces two equal branches. . However, the main stem exhibited unequal branching (called overtopping (text with tooltip) Overtopping is a type of growth pattern in which there is unequal dichotomous branching. This produces a larger stem and a smaller lateral stem. ) in a pseudomonopodial (text with tooltip) Pseudomonopodial growth occurs in plants that exhibit weak overtopping. mode. Thus, the trimerophytes appeared more plant-like with a larger main stem and branches smaller lateral branches that terminated in clusters of sporangia (or smaller branches) that looked somewhat leaf-like. Some of them like Psilophyton (Figure 1) were covered with pegs (like the zosterophyllophytes). The steles of the main branches were protostelic and centrarch (text with tooltip) Pseudomonopodial growth occurs in plants that exhibit weak overtopping. or mesarch (text with tooltip) A mesarch (adj) stele is one in which the protoxylem forms between the center and outer part of the xylem. . These plants flourished during the Devonian period but became extinct at its close.

SYSTEMATICS OF THE TRIMEROPHYTOPHYTA The trimerophytes were defined as a group by Banks (1975). They have been considered to be group of pre-megaphyllous vascular plants in the line leading to ferns (e.g. Bold et al. 1987). Kenrick and Crane (1997) and Doyle (1998) take the approach that the trimerophytes are basal euphyllophytes (see Figure 2), the plants that bear true leaves or their precursors. As a group, the euphyllophytes also include the living groups: ferns, and seed plants. We have taken the cautious approach of Bold et al. (1987) and kept the trimerophytes as a phylum (=division).

FIGURE 1. Reconstructions of Trimerophyton (left) and Psilophyton (right). Both show overtopping and terminal dichotomously-branched stems. Image from: Northington and Goodin (1984)

FIGURE 2. Major Clades of the Vascular Cryptogams. P/St = polysporangiate and vascular stele O/M = overtopping and microphyll E = euphyllophyte SS = stalked sporangia

LITERATURE CITED Banks, H. P. 1975. Reclassification of Psilophyta. Taxon. 24: 401-413. Bierhorst, D. W. 1971. Morphology of Vascular Plants. In: N. H. Giles and J. G. Torrey. The MacMillan Biology Series. The MacMillan Co. New York. Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. Doyle, J. A. 1998b. Phylogeny of vascular plants. Annual Review of Ecology and Systematics. 29:567-599. Kenrick, P. and P. R. Crane. 1997b. The Origin and Early Diversification of Land Plants: A Cladistic Study. Smithsonian Institute Press. Washington, DC. Northington, D. K. and J. R. Goodin. 1984. The Botanical World. Times Mirror/Mosby College Publishing, St. Louis.

By Jack R. Holt. Last revised: 03/25/2013