Functional Aspects of the Origin and Subsequent Evolution of Cotyledons in Seed Plants

Abstract

In most seed plants, cotyledons formed within the seed act as haustorial organs, as well as playing a key role in releasing the shoot apex from the seed coat on germination. Emergence of the shoot apex often results from asymmetric intercalary growth of the cotyledon bases. This process avoids the principal spatial constraint on germination in seed plants, wherein the plumule is surrounded by cotyledons whose tips should be in contact with nutritive tissues. Cotyledons are commonly viewed as a synapomorphy of the seed-plant lineage and as the modified first leaves of a seedling. However, intercalary growth, which is crucial for the cotyledon’s role in releasing the plumule, had not yet been acquired by foliage leaves of the earliest seed plants. We explore the possibility that the program of intercalary growth in leaves first evolved in cotyledons and was subsequently recruited for foliage leaves in the seed-plant lineage. We also discuss whether early seed plants already possessed haustorial cotyledons.