The trimerophytes demonstrate more complex morphology and anatomy than rhyniophytes, their presumed ancestors, although both groups are coeval. In the trimerophytes, plant architecture is monopodial or pseudomonopodial. Laterals are produced in a variety of patterns, including helical (Psilophyton sterile branches), alternately and distichous (Psilophyton fertile branches), tristichous (Trimerophyton), and tetrastichous (Pertica). In Pertica the ultimate branch-lets consist of slender, three-dimensional dichotomizing structures. It has been suggested that the planation of these lateral branches would provide the morphologic equivalent of a megaphyllous leaf and that Pertica may be used as a transitional morphotype in the evolution of a frond or a leaf. In another group of Devonian plants, the Aneurophytales
(progymnosperms), some taxa possess planated laterals, whereas in others the branching systems are more three dimensional (Chapter 12).
Trimerophytes also demonstrate various patterns of sporang-ial attachment. In rhyniophytes, sporangia are terminal at the ends of dichotomizing axes. In some species of Psilophyton the number of sporangia is small, while in others (e.g., P. dap-sile) numerous small sporangia are clustered together. One possible transformational series might involve Pertica, with its massive clusters of densely packed sporangia, leading to some Carboniferous ferns, such as some species of Botryopteris. Another line might lead to the progymnosperms through such a plant as Tetraxylopteris. One Middle Devonian plant that might bridge the evolutionary gap between trimerophytes and progymnosperms is Oocampsa (Andrews et al., 1975). Specimens of O. catheta consist of closely spaced, helically arranged branches up to 7.0 cm long that were produced from a primary axis (FIG. 8.86). The lateral branches divide pseu-domonopodially and dichotomously and terminate in elongate, erect sporangia. Sporangia dehisce longitudinally and contain
large (96-120 pm), trilete miospores. The spores are interesting in that there appears to be some space between the wall layers, suggesting a pseudosaccate morphologic type. Oocampsa , with erect sporangia borne on helically arranged primary and secondary branches, may be transitional between certain trimerophytes, for example, Trimerophyton and Pertica , and Tetraxylopteris, a progymnosperm with pinnate arrangement of ultimate segments. Such a series is congruent with the strati-graphic occurrences of the taxa listed.
There are relatively few trimerophytes that are structurally preserved. In Psilophyton dawsonii and P. princeps , there is a simple conducting strand, but one that is more massive than any known for the rhyniophytes. Conducting elements in the rhyniophytes are of the S-type, while Psilophyton contains P-type elements in which secondary wall material is deposited between the scalariform bars.
Although the trimerophytes appear to be less diverse than other early land plants, there are some apparent evolutionary trends within the group, including stages in the evolution of a particular type of leaf, modification of conducting element pitting toward the circular-bordered type, and some suggestion of an early stage in the evolution of spores with an increased surface area as a result of the separation of wall layers. As additional specimens are discovered and described from Devonian or possibly even Mississippian rocks, it is obvious that this group will play an increasingly important role in our understanding of levels of specialization in early land plants and their role in the diversification of later-appearing groups.
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