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Palms and the Origin and Evolution of Monocotyledons

Past and Present taxonomic treatments and evolutionary analyses of monocotyledons have noted that unspecialized reproductive structures occur in some palms, but the size, inaccessibility, apparent and often real complexity of these plants have hampered detailed studies until recently. The most primitive palms are 14 or 15 apocarpous genera belonging to the Trithrinaux alliance in the coryphoid group. Apocarpy also characterizes Phoenix, a genus related to coryphoid palms but more specialized in some respects, and Nypa fruticans, a mangrove palm with a unique combination of primitive and specialized features. Distribution patterns, habits, leaf structure, germination, reproductive structures, chromosomes numbers, vessels, and vascular systems are described, as presently known, in these primitive palms, and their significance in the evolution of other palms, monocotyledons, and dicotyledons is evaluated. It is concluded that the palsm are not a morphologically unique and isolated group, as often postulated, but that they frequently express most clearly evolutionary sequences fundamental to monocotyledons as a whole. Evolution in palms substantiates a number of different evolutionary sequences in monocotyledons and dicotyledons, and suggests other possibly different trends for investigation. The review provides evidence that the exploitation of different evolutionary sequences by monocotyledons and dicotyledons necessitates separate consideration of evolution within each class of angiosperms. It is suggested that an early divergence of monocotyledons and dicotyledons from protoangiosperms or perhaps even from preangiosperms may be compared to the evolution of birds and mammals from diverse reptilian groups.