VIII. A Contribution to the Study of the Evolution of the Flower, with special reference to the Hamamelidaceæ, Caprifoliaceæ, and Cornaceæ.

Abstract

Summary 1 The following details of floral morphology are recorded:— a.  The occasional occurrence of hermaphrodite flowers in Aucuba himalaica. b.  Biovulate ovaries in A. japonica and himalaica. c.  Anomalous flowers in the hybrid Garrya Thuretii—these consist of three congenitally united flower-heads borne upon one “pedicel,” thus combining the characters of both parents, viz., G. elliplica in which the flowers occur in axillary trimerous groups, and G. Fadyenii in which they are situated singly in the axil of each bract. d.  Rudiments of a single whorl of stamens in the female flowers of Griselinia littoralis and lucida and of the absent petals in G. lucida. e.  Rudimentary terminal parietal placentae in Corokia Cotoneaster. 2 The terminal ovules are suspended axially in Davidia, Cornus, Helwingia, and in bilocular forms of Marlea and Corokia; probably axially by origin in Nyssa and Griselinia, and parietally borne in Aucuba and Garrya. 3 The ovule exhibits considerable variety in organization:— a.  It is in Nyssa and Davidia of the dorsal type (raphe ventral or adaxial) with radial orientation; of the ventral type in Helwingia and Corokia (raphe dorsal or abaxial) with radial orientation, but the inverse of the former; and of the ventral type with tangential orientation in bilocular forms of Cornus and Marlea. b.  The nucellus in the genera investigated (including Davidia and Nyssa described as bitegumentary by Harms and Wangerin) becomes invested with a single integument. c.  The integument is completely free from the nucellus in Aucuba and Griselinia, but in the specialized ovules of Davidia and Helwingia is only found as a separate organ towards the summit of the nucellus. d.  The nucellus consists of about seven axial rows of cells in Aucuba; four or five in Cornus, Davidia, Griselinia, and Garrya; and three in Helwingia, where it approximates to the condition obtaining in Lilium and Inula. e.  The nucellus of Aucuba, in the formation of a definite layer of parietal tissue, a small epidermal cap, and in subsequent bulk-extension owing to cell-division, approaches the type described by Pechoutre for the Rosaceae. f.  The germination of more than one megaspore, already recorded by the writer for Davidia, occurs also, but not to the same extent, in Aucuba. 4 The floral vascular systems of the Cornaceae do not conform to one type as in the Caprifoliaceae. It is in Cornus of the Caprifoliacean type. The systems of Griselinia, Nyssa, and Helwingia exhibit Araliacean tendencies; those of Davidia and Marlea are transitional in character between that found in Aralia and the synthetic systems of Cornus and the Caprifoliaceae. The stage of economical advance reached by Viburnum has not been attained. a.  a. Infra-locular extensions of vascular tissue occur in Aucuba and unilocular forms of Marlea. b.  The ovular trace is anomalous in Griselinia, Nyssa, and unilocular forms of Corokia. c.  Branches from the ovular trace traverse the peripheral region of the integument in Davidia and Corokia. 5 The flowers of certain genera admit of the following interpretation:— a.  Cornus: The ovary is biloeular by reduction from a quadricarpellary primitive type. It is analogous to that of Sambucus. b.  Alangium: The ovary is unilocular by reduction from the bilocular type found in Marlea begonifolia, and reduced from a form primitively polycarpellary. c.  Aucuba: The flowers have become unisexual since the attainment of epigyny. The ovary was primitively quadricarpellary and quadrilocular. The septa subsequently disappeared, but the lower portion of the carpels remained unmodified. The ovules, originally numerous, became numerically reduced until only a single parietally suspended one remained. d.  Garrya: The flower is hypogynous as already noted by Wangerin. e.  Griselinia: The flowers have become unisexual since the attainment of epigyny, and a tendency towards apetaly finds its expression in G. lucida. The ovary was primitively tricarpellary and trilocular, at all events in its upper portion, but the septa disappeared, leaving the lower portion of the carpels unmodified, but involving the displacement of an ovule that was primarily axially situated. 6 Progressive abortion in the ovary reaches its limit in this family—the ovaries of Davidia and Helwingia are multilocular, the change from the bi-to the unilocular condition can be traced in Corokia, and the ovaries of Aucuba, Nyssa, Alangium, and Griselinia are unilocular. 7 Progressive sterilization has proceeded still further than in the Hamamelidaceae and Caprifoliaceae, nearly all the genera have reached the level of the extreme cases in these families, and the ovary or loculus is absolutely uniovulate. 8 The general series that has been traced in the direction of the loss of carpellary autonomy culminates in Aucuba, where, owing to the absence of septa and to the parietal situation of the ovule, the gynaecium appears to consist of a single carpel. 9 Both the Cornaceae of Harms and the Cornaceae of Wangerin are heterotypic, but this fact is masked, owing to parallel development and convergent organization.