The Bases of Angiosperm Phylogeny: Floral Anatomy

Abstract

An eclectic ramble through phylogenetic aspects of floral structure includes the following: (1) Sterling's view that the ancestral flowers of Rosaceae had only two ovules per carpel is examined and rejected. (2) Recent observations on the direction of androecial development in various taxa are reviewed, and it is concluded that centrifugality is not as valuable a phylogenetic indicator as some systematists had hoped it would be. (3) An attempt is made to reinterpret the inverted placental bundles of Capparales and the inverted recurrent bundles of Nestronia along morphogenetic lines. It is suggested that the inverted orientation is causally related to the initiation and differentiation of these bundles in isolation from previously formed vascular tissue. Floral anatomy turns some botanists into fantasts, others into iconoclasts. But despite the frequent speculative excesses, the occasional overreaction, and the recurring disagreements that are a part of the field, serially sectioned and cleared flowers continue to provide essential phylogenetic information. To begin with a straightforward example, consider Cronquist's (1968) suggestion concerning the origin of the Proteales, which he defines as Proteaceae plus Elaeagnaceae. Stressing similarities between the Proteales and the Thymelaeaceae (a point of difference with Takhtajan, 19702), Cronquist postulates that the origin of the order was in the Myrtales. For this to be true, the gynoecium in Proteaceae and Elaeagnaceae must be pseudomonomerous; in other words, it must be a syncarpous gynoecium that has acquired through evolutionary processes the superficial appearance of a single carpel. Noting that the Myrtales, which are syncarpous, must be excluded as possible ancestors if the gynoecium of the Proteales should turn out to be a solitary carpel, Cronquist adds: most likely origin of the Proteales would then be in the -Rosales. Serial cross sections through the gynoecia of various Proteaceae make Cronquist's favored position for the Proteales untenable, for there is no sign of pseudomonomery. Instead, each gynoecium has the three major vascular bundles and the ventral suture of a single carpel (Fig. 1). The same is true of Elaeagnaceae (Fig. 2; see also Eckardt, 1937: 47). The ancestry of the Proteales must therefore be sought in a group with apocarpous members such as Cronquist's Rosales or Takhtajan's Saxifragales. The conviction that an apocarpous gynoecium did not originate from a syncarpous gynoecium will not be challenged in this forum because evidence is overwhelming that apocarpy preceded syncarpy in many groups of flowering 'Department of Botany, Smithsonian Institution, Washington, D.C. 20560. 2 Icite the Russian version of Takhtajan's Flowering Plants: Origin and Dispersal ( 1970) rather than the English version (1969). Although the English version was translated from a Russian manuscript, the printed Russian version appeared later and differs in a number of ways (see, for instance, the newly segregated families in Cornales). While my symposium contribution awaited publication, Fischer Verlag published a German version: Evolution und Ausbreitung der Blfitenpflanzen (1973). ANN. MissouRi BOT. GARD. 62: 521-537. 1975. This content downloaded from 157.55.39.246 on Tue, 27 Sep 2016 05:53:31 UTC All use subject to http://about.jstor.org/terms 522 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 62