Seed weight in Fabaceae ranges over seven orders of magnitude from 0.1 mg to 334 g. The frequency distribution is strongly right skewed with a single modal peak at 18 mg. The two sets of morphological characters, the presence or absence of endosperm and the anatomical form of the cotyledons, interact to define eight structurally and functionally distinct seed types. The predominant seed type is endospermic with foliar cotyledons. The other seed types have evidently arisen from it by transfer of storage function from endosperm to cotyledons, culminating in exendospermic seeds with storage cotyledons. This has probably happened repeatedly in disparate tribes and genera throughout the family. There is a highly significant allometric relationship between the logarithms of the weight of nitrogen in the seed and the total seed weight, with the datasets for nodulating and non-nodulating species yielding significantly different regression equations. The rate of increase in weight of nitrogen is lower than the rate of increase in total seed weight, and this discrepancy is greater in non-nodulating than in nodulating species. A consequence of the discrepancy is an overall inverse relationship between seed weight and nitrogen concentration. Seeds of the same type are significantly larger and have a higher absolute nitrogen content, but a lower nitrogen concentration in non-nodulating than in nodulating species. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 167, 251–280.