XIV. The Structure and Affinities of Davidia involucrata, Baill.

Abstract

Summary. 1 The vascular structure of the hypocotyl varies considerably. 2 The inflorescence consists of a number of congenitally-fused, apetalous, multi-staminate male flowers: or of male flowers and, in addition, a single obliquely-situated, apetalous hermaphrodite flower with epigynous stamens arranged in series. The arrangement in series is suggested on account of the vascular attachment of as many oppositilocular stamens as there are loculi to the ovarian cylinder of bundles. 3 The insertion of the bract-trace takes place above the point of insertion of the bracts themselves, and sometimes as high up as the torus. There are, therefore, three vascular “bundles” on each side of the axial cylinder in the peduncle above the insertion of the bracts. 4 The ovules in the multilocular ovary—seven being the mean number of loculi—are attached axially, one in each chamber, and unaccompanied by a rudimentary ovule: correlated with this the vascular bundle in the raphe is made up of two bundles, one from each adjacent septum, joined together, and not of a single bundle from an adjacent placenta, as is frequently the case in the Araliaceae and Umbelliferse. 5 The ovule is of the Aralian type, only more specialized, possessing a single, very incomplete integument, which grows up leaving the micropyle on one side or the other. 6 The nucellus is of small bulk and comparable to the nucelli of Cornus and Aralia. It does not persist. 7 The sporogenous tissue usually gives rise to more than one embryo-sac, of which only one develops to any extent. A number of abnormalities occur in the number, arrangement, and form of the embryo-sac nuclei. The functional embryo-sac of the only fertile ovule observed was perfectly normal. Sterilization processes in the Caprifoliacese are accompanied by abnormal nuclear phenomena in the embryo-sacs of sterile ovules (Symphoricarpus), and by the presence of a multicellular archesporium in the nucelli of incomplete ovules (Viburnum). In the Rubiaceae highly specialized ovules are associated with a multicellular archesporium. Under the circumstances no primitive significance is attached to the condition of the archesporium in Davidia. 8 After fertilization time, the tissues of the congenitally-fused integument and raphe (extra-nucellar tissue) differentiate into an inner meristematic zone including the secretory epithelium (internal epidermis), and an outer non-meristematic zone. The expansion of the narrow cylinder of meristem, increasing in size mainly by cell-division in transverse or obliquely transverse planes, causes considerable increase in length over girth during cell-enlargement, and has a dragging effect on the non-meristematic cells, so that previously transverse cortical rows arch downwards as if they had been drawn down toward the axial cylinder, and the seed becomes almond-shaped. The vascular bundle branches considerably; the fine capillaries, which do not penetrate inward, end in the abraphal side. 9 During the development of the primary embryo the old antipodal system disappears. The cells of the internal epidermis are organized into a definite epithelium which forms a means for supplying the sac uniformly with nutritive material, after the manner described by Billings for the Caprifoliaceae. The extra-nucellar tissue forms a kind of water-jacket, the inner cells of which (inner, meristematic zone) function as collecting-cells. 10 True endosperm is present as a thin-walled tissue in very young stages of the primary embryo. Owing to intermittent division of the endosperm cells and the general elongation of the sac, the outline of the cells is irregular. 11 The upper cells of the young pro-embryo function as a temporary absorbent organ, leaving a bulbous cavity in the endosperm when they disappear. By this time the true embryo begins to develop at the end of the suspensor as a small undifferentiated mass of cells, and later takes upon itself the absorptive function of the suspensor. 12 The fruit at the time of germination consists of an exceedingly hard stony endocarp formed by the extreme sclerosis of all the ovarian tissue within, and including that of the inner peripheral cylinder of bundles. Dehiscence takes place by the removal of the upper part of the back of each chamber. The seeds are enveloped in a brown, scaly testa, which is all that remains of the extra-nucellar tissue. 13 The embryo gradually absorbs the endosperm by means of its epidermal layer, which, at the time, functions as a secretory epithelium. During germination the radicle emerges from the endosperm in the manner described by Hemsley. 14 From a detailed study of the flowers, ovary, ovule, and seed, the author is inclined to believe that Davidia has no real affinity with the Hamamelidaceae, but is distantly related to Alangium and Nyssa, and still more distantly related to the Araliaeese: that the genus occupies a more isolated position than has hitherto been supposed, owing to having pursued an independent course of development from the plexus of primitive groups which included the ancestral forms of the Araliaceae, Nysseae, and Alangieae.