Of several theories for the origin of the ovule advanced in this century and based largely on fossil evidence, the telomic concept is widely considered the most plausible. Its principal tenet is the evolution of the integument through fusion of sterile branches or teiomes around a terminal megasporangium. The only point of agreement in these theories is that the entire nucellus is a megasporangium that retains a single megaspore and the endosporic female gametophyte. Their differences center on the origin of the integument. A new concept offered here on evidence from ovules of both Paleozoic and extant seed plants significantly alters the telomic theory. It proposes that the nucellus is a sporangiophore of stem origin that bears a terminal megasporangium; that at least some of the fused integumentary telomes were fertile; and that among all features cited to characterize ovules, the unique nature of the retained megaspore alone defines the seed habit. Changes in the seed plant megaspore that extended the period of nutrient absorption over the whole course offemale gametophyte development, along with complex physiological changes in the nucellus, were probably achieved along a single phylogenetic line beginning in a Late Devonian population of progymnosperms. For such a combination of events to have occurred more than once is highly unlikely, and, therefore, a monophyletic origin for seed plants is proposed. Several primitive features in ovule structure, some not evidenced since the Lower Carboniferous Period, occur in a mutant form of Arabidopsis thaliana isolated from genetically transformed plants. Their recurrence provides additional support for the proposed concept of ovule origin and also suggests that the genetic mechanisms for expression of primitive features in advanced taxa could be initiated in each case by mutation of a single homeotic gene.The ovule, a salient feature of gymnosperms and angiosperms, is traditionally described as a megasporangium or nucellus surrounded and protected by one or two integuments. Usually, the megasporangium retains a single megaspore, which through endosporic germination produces a female gametophyte. With syngamy, the mature ovule becomes an immature seed, and the seed matures as the zygote produces the embryo. Seeds thus represent the endpoint ofovule ontogeny. Previous studies on ovule origin and the beginning ofthe seed habit, focused largely on evidence from fossil gymnosperms and arborescent lycopods, usually apply the terms ovule and seed interchangeably, since a clear distinction is seldom revealed. Considerable variation in the interpretation of fossil evidence has led to several theories for ovule origin, but an unquestioned advocacy for anyone over all others has not appeared. This report will recount past theories for ovule origin, examine the presumptions for some interpretations ofthe fossil record, and present evidence from developmental studies of extant seed plants previously not considered for this issue. The preponderance of evidence presented supports a...