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trimerophytes A group of extinct land plants that flourished during the latter part of the Devonian period (380–360 million years ago), members of which evolved into the ancestors of seed plants and ferns. They were robust herbs and small shrubs, some up to 3 m tall, and showed significant changes in branching pattern compared with their predecessors, the rhyniophytes. Instead of equal (dichotomous) branching, one stem tended to be dominant and give rise to smaller lateral branches, some bearing spore-forming organs and others acting as leaves. Moreover, the branching pattern became more regular. This trend culminated in such forms as Pertica quadrifolia, a plant with an obvious main trunk and numerous small lateral branches.