Morphological and Ontogenetic Studies on Torreya californica. II. Development of the Megasporangiate Shoot Prior to Pollination

Abstract

Kemp, Mahgaret. (Smith Coll., Northampton, Mass.) Morphological and ontogenetic studies on Torreya californica. II. Development of the megasporangiate shoot prior to pollination. Amer. Jour. Bot. 46(4): 249–261. Illus. 1959.—The development of the megasporangiate of Torreya californica during the first part of its maturation cycle of 26 months is described in detail. This first developmental period extends from the initiation stage in late July of one year through pollination of the young ovules in April of the following spring. At the end of this period, the reproductive shoot is a loosely organized, compound, determinate structure. It consists of a short primary axis which originated in the axil of one of the last formed bud scales or one of the first formed foliage leaves of the vegetative bud. This primary axis bears only 2 lateral and oppositely placed prophylls which stand at right angles to the subtending structure. In the axil of each prophyll is a short secondary axis which bears 2 successive pseudodecussate pairs of subopposite, sterile, scale-like perianth segments below a solitary, erect, terminal ovule. The integument of the ovule originates as a single lateral primordium, but its margins quickly merge and at pollination time it is a tubular envelope free from the nucellus. The nucellus, which is massive and contains a single deeply imbedded megasporocyte, terminates the secondary axis. Histogenetically, both primary and secondary axis systems of the megasporangiate shoot resemble a vegetative dwarf shoot. They both originate as axillary mounds of uniformly meristematic cells, whose apices soon exhibit a zonal pattern comparable to that of the apex of the vegetative shoot of the same species. The determinate nature of the primary axis is caused by cell senescence in its apex. The prophylls of the primary axis and the perianth segments of the secondary axes are comparable to bud scales of the vegetative bud in their arrangement, their origin from subsurface layers, the presence of apical and subapical initials which produce their first vertical growth, a basal intercalary meristem which completes their elongation, and marginal initials which produce a slender wing to the lamina of each type of cataphyll. At maturity all 3 types of cataphylls are basically similar in their histology. The apex of each secondary axis, at the initiation of the integument, shows an altered cellular pattern which rapidly becomes organized into a conspicuous fanshaped coaxial system as the central portion develops directly into the massive, cauline nucellus. This coaxial apical configuration differs markedly from the zonal pattern of the vegetative shoot apex and also from the similar zonal pattern in the apex of the primary axis of the megasporangiate shoot and its secondary axes during an earlier period of indeterminate growth.