Megasporogenesis and embryo sac development in Excentrodendron hsienmu (Malvaceae) with reference to the systematic affinities of Excentrodendron

Segregation of Excentrodendron and Burretiodendron has hardly been accepted despite their distinct pollen morphology. Megasporogenesis and embryo sac development were studied in Excentrodendron hsienmu in order to examine the basis of segregation of Excentrodendron and Burretiodendron and to study the affinity of Excentrodendron. Excentrodendron hsienmu is dioecious and a large number of flower buds developed every year on mature male and female trees but flower blossoming and fruiting did not occur every year and there was a cycle of 3-4 years. Staminate and pistillate flowers were not distinguishable in the early development stages since both microsporogenesis and megasporogenesis were observed in staminate and pistillate flowers, respectively. Megasporogenesis in staminate flowers degenerated after demarcation of megaspore mother cell or rarely after megaspore tetrads. The monosexual flowers of Excentrodendron were probably derived from bisexual flowers. The ovule was anatropous, bitegmic, and crassinucellate. The primary archesporium of ovules consisted of usually one cell and integument initials were not demarcated at the time of the differentiation of the archesporium. The outer and inner integument of the mature embryo sac was 3-4 and 3 cells thick, respectively. The megaspore mother cell was usually deeply seated in the nucellus before meiosis and gave rise to a linear or rarely T-shaped tetrad. The development of embryo sac conforms to the Polygonum Type. Degeneration was observed commonly after demarcation of megaspore mother cells; 88% of embryo sacs degenerated and only 9% of ovules developed to seeds. Excentrodendron shares with Burretiodendron a number of plesiomorphic characters that are commonly found in members of Malvales. It differs from Burretiodendron mainly in the thickness of the inner integument which is 3 cells thick in Excentrodendron but 4-5 in Burretiodendron. In addition, archesporium is usually unicelled in Excentrodendron but multicelled in Burretiodendron. Segregation of Excentrodendron is therefore supported by the present study. Embryologically, Excentrodendron differs also from Pterospermum, Dombeyeae, and other taxa of the expanded Malvaceae, mainly in the thickness of integuments and number of archesporial, cells. The present study does not support the placement of Excentrodendron in the tribe Dombeyeae of Malvaceae s.l.