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The bennettites are an extinct group of gym nosperms—seed-bearing plants whose seeds are exposed to the air, not enclosed in the ovary of a flower. Botanists hypothesize that bennettites are related to the cycads, an extant group of gymnosperms, and paleobotanists believe the bennettites originated from the seed ferns (Pteridospermales) about 220 million years ago during the Triassic period. Bennettites became extinct in the Upper Cretaceous period, about 100 million years ago.

Bennettites had palm-like leaves with stems that were thin and branched in some species, and stout and trunk-like in others. Most species had stems with a large central pith. The bennettites are also distinguished by certain microscopic features of their guard cells. Guard cells are specialized cells on the surface of a leaf that regulate opening of stomata (pores) in the leaf for photosynthetic gas exchange. The bennettites had guard cells with a large amount of cutin, a naturally occurring plant wax.

The best-known genus of the bennettites is Cycadeoidea. Knowledge about this genus has been gleaned mostly from fossils found in the Black Hills of South Dakota. Many of these fossils were collected and studied in the early 1900s by George R. Wieland. Wieland proposed that the strobili (reproductive structures) of Cycadeoidea functioned like flowers and thus that this genus was a close ancestor of the angiosperms, the flowering plants. More recent evaluation of these and other fossil strobili of the bennettites, however, indicates that they differed significantly from angiosperms. It has been shown instead that bennettite strobili were bisporangiate (containing male and female reproductive organs in the same structure) and that they probably relied on self-pollination to reproduce.

See also Paleobotany.

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