Introduction

The goal of this chapter is to review seedling morphology and evolution within a broad phylogenetic perspective. Its contents, therefore, are derivative of numerous and widely scattered publications.

Traditionally, seedling refers to the juvenile seed plant sporophyte after its emergence from the seed coat, which immediately evokes the concept of the seed itself, that is, an indehiscent, integumented megasporangium (Bierhorst, 1971; Gifford & Foster, 1988). Within this limited phylogenetic framework, a review of the morphology and evolution of the seedling is necessarily restricted to the seed plant lineages (i.e. spermatophytes) represented in contemporary floras by Ginkgo biloba, cycads, gnetophytes, conifers, and angiosperms.

However, the thesis advocated in this chapter is that a much broader phylogenetic perspective is required to understand seedling morphology and evolution fully because many of the features that characterize the seedling sensu stricto evolved well before the appearance of the first seed plants. Perhaps the most important of these features is the physical retention and physiological nurturing of the developing sporophyte within gametophytic tissues. This feature is characteristic of all land plants (i.e. embryophytes) by virtue of their (1) diplobiontic life cycle, in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte to complete the sexual reproductive life cycle, and (2) their archegoniate condition, in which the developing diploid embryo is retained, protected, and nurtured within an archegonium (or its presumed vestigial remnants, e.g. synergids).