Abstract Detail

Anatomy and Morphology

Kwon, Sarah [1], Hayden, W. John [2].

Floral Anatomy and Morphology of Acalypha setosa (Euphorbiaceae).

Acalypha setosa is a weedy herbaceous plant native to the New World tropics and found sporadically through the southeastern United States. This species produces three distinct flower forms: staminate flowers on short axillary spikes, pistillate flowers on terminal spikes, and allomorphic pistillate flowers at the distal extremity of staminate or pistillate spikes, and in leaf axils, adjacent to bases of staminate spikes. While some previous literature exists on anatomy of pistillate flowers in Acalypha, internal structures of staminate and allomorphic flowers have received little attention. We studied all three flower forms via light and scanning electron microscopy. Staminate flowers consist of eight stamens enclosed by four sepals; filaments arise from a central column. Anthers are elongate, convoluted in bud, and helically coiled during the brief early morning period of anthesis—a detail not noted in previous literature; tapetum is amoeboid; endothecium dominates mature anther walls; and stomium consists of paired large and small cells on each side of the opening. Pistillate flowers consist of three small sepals nearly hidden beneath the three-carpellate gynoecium. Ovaries are superior and styles are divided. Each locule bears a single pendulous, anatropous, ovule; at anthesis the nucellar beak extends beyond the outer integuments and inner integuments extend only to the midpoint of the megasporangium; further the two integuments are well-separated from each other and from the outer surface of the megasporangium. We interpret allomorphic pistillate flowers to originate as a three-carpellate gynoecial primordium from which only one carpel expands via an allometric pattern of growth. The axial portion of the developed carpel does not expand while the dorsal midline (carpel midvein) does, ultimately causing that carpel to assume a cylindrical outline, which is enhanced by growth of cells adjacent to the dorsal midline. Mature allomorphic flowers consist of a central crest (the dorsal midvein) flanked by a pair of toothed, crown-like, projections, with styles emerging from the base of the crest. We observed embryos within fruits developing from allomorphic flowers. Staminate sepals and styles of both pistillate and allomorphic flowers possess distinctive crystal-bearing papillae. We hypothesize that our allometric model for allomorphic flower development in A. setosa may also be a significant component of the ontogenetic processes found in allomorphic flowers of other species in the genus.

1 - University of Richmond, Department of Biology, 28 Westhampton Way, Department of Biology, Richmond, Virginia, 23173, United States
2 - UNIVERSITY OF RICHMOND, DEPT OF BIOLOGY, 28 WESTHAMPTON WAY, RICHMOND, VA, 23173, USA

Keywords:
Acalypha
floral anatomy
staminate flower
allomorphic flower.

Presentation Type: Oral Paper
Session: 20, Anatomy and Morphology
Location: Fort Worth Ballroom 7/Omni Hotel
Date: Tuesday, June 27th, 2017
Time: 8:30 AM
Number: 20003
Abstract ID:300
Candidate for Awards:Maynard F. Moseley Award