Inflorescence Ontogeny and Floral Organogenesis in Grevilleoideae (Proteaceae), with Emphasis on the Nature of the Flower Pairs

Abstract

We examined and compared floral and inflorescence ontogenies among taxa of the subfamily Grevilleoideae in Proteaceae to clarify the diverse orientations of flowers in the subfamily and to define the developmental basis of the flower pairs, i.e., the presence of two flowers in the axil of a common bract on an inflorescence axis. To accomplish this, we compared the earliest inflorescence and floral developmental stages of 50 species representing 32 genera and later developmental stages of an additional 18 species and 14 genera using morphological developmental techniques. There is a common and conserved developmental pathway involved in the initiation of the flower pairs among all of the taxa: first-order axillary meristems are initiated, each one in the axil of a common bract on a principal inflorescence axis; the axillary meristem enlarges and is termed a short-shoot meristem; two floral bracts develop, one on each side of the short-shoot meristem, in each of which a floral meristem will enlarge; and there is a persistent short-shoot residuum between the two floral meristems. The flower pairs are redefined as two-flowered short shoots. Hypotheses regarding the origin of the flower pairs are evaluated within a developmental context, and there is support for the hypotheses that the short shoots are derived from reduction of secondary inflorescence axes or from amplification of a first-order axillary meristem. There is some variation at all developmental levels of the flower pairs, including differential enlargement of the short-shoot meristem (most are disymmetrical, some become concave-arcuiform), the timing and position of initiation of floral bracts as well as their subsequent enlargement or suppression, the successive timing of initiation of the four tepals, and aestivation patterns (most exhibit sagittally incomplete valvate tepals). On the basis of this ontogenetic study, the orientation of the flowers of Grevilleoideae appears highly conserved; each flower of a pair is dorsiventrally aligned and subtended by a floral bract (in most taxa) and on a short-shoot axis. The high degree of conservation of floral orientation among Grevilleoideae will make possible explicit hypotheses of homology of floral characters among taxa both within and outside the subfamily, an essential precursor to a phylogenetic analysis.