J Syst Evol ›› 1990, Vol. 28 ›› Issue (6): 430-441.

• Research Articles •

### The Anatomy, Embryology and Systematic Relationships of Eucommiaceae

Zhang Zhi‐Yu, Lu An‐Ming, Pan Kai‐Yu, Wen Jie

• Published:1990-11-10

Abstract: In this work examined were leaf and wood anatomy and embryogenesis under LM and pollen morphology under SEM of Eucommia ulmoides Oliv. The results were used for a comparison between the family and Ulmaceae and Hamamelidaceae respectively. The taxonomic rank and relationships of E. ulmoides were analyzed mainly based on the spiral thickenings on lateral walls of vessels in the secondary xylem, the presence of iridoid, embryology and palynology. 1. The present authors tend to support Keng's (1962) view that the spiral thickenings on lateral walls of vessels are the remnant of a primitive character. The spiral thickenings on lateral walls of vessels in E. ulmoides (Plate 1: 9) are similar to those of some genera of the Hamamelidaceae (e. g. Altingia Noronha), while vessels in Ulmaceae lack spiral thickenings on lateral walls. The Eucommiaceae with simple perforations plates (Plate 1: 9) is more specialized than the Hamamelidaceae. 2. Based on the fact that the Eucommiaceae contains iridoid compound and has unitegmic ovules and cellular endosperm, Dahlgren (1980, 1983) places with uncertainty the family in Corniflorae as an order, a treatment which is widely discrepant from those of Takhtajan (1980), cronquist (1981) and thorne (1983). Though containing iridoid compound, the Eucommiaceae is different from Corniflorae in a combination of characters in external morphology, woody anatomy and embryology. The compound has also been found in Liquidambar L. (Hamamelidaceae) but not in the Ulmaceae, which is another piece of evidence showing a close relationship between Eucommiaceae and Hamamelidaceae. 3. The development of microsporangia and megasporangia, as observed in the present work, is basically in accordance with that reported by Tang (1962) and Eckardt (1963), but the haustoria present both at the micropylar end and at the chalazal end and 4-celled proembryo of the Solanad Type are reported here for the first time. It can be seen from Table 2 that the Eucommiaceae and the Hamamelidaceae have a number of embryological characters in common For example, glandular tapetal cells in anthers are usually multinuclear; cytokinesis of meiosis of pollen mother cells is simultaneous; microspores develop into tetrahedral; ovules are anatropous, crassinucellate; embryo sacs are of the monosporic Polygonum Type; endosperm is cellular (Plate 3: 5-7; 4: 1-3; 5: 1-3; Fig. 1: 1-3,5). The Eucommiaceae is also embryologically related to the family Ulmaceae, but the family under study is more specialized than the two families mentioned above in unitegminy (Plate 4: 3,4), proembryo of the Solanad Type (Plate 6: 3; Fig. 2: 4), coexistence of micropylar haustoium (Plate 6: 4-6) and chalazal hustorium, especially in the Eucommiaceae the epidermis and the endothecium are widely separate (Plate 3:3), a feature which has never been seen in angiosperms to our knowledge. 4. Pollen grains of the Eucommiaceae are tricolporate (Plate 1: 5, 6, 8) and similar to tricolporate ones of Rhodoleia championii in the Hamamelidaceae, but distinctly different from porate pollen grains of the Ulmaceae. 5. Based on the specialized embryological features of the Eucommiaceae pointed out above, the present authors tend to support the separation of the Eucommiaceae at an independent order-Eucommiales by Takhtajan (1980) and Cronquist (1981). Considering the spiral thickenings of vessels on lateral walls and the presence of iridoid in the Eucommiaceae, and the similarities and differences in embryology and palynology among Eucommiaceae, Hamamelidaceae and Ulmaceae, the authors suggest that the Eucommiaceae is more closely related to the Hamamelidaceae than to the Ulmaceae, and postulate that the Ulmaceae (in Urticales)and the Eucommiaceae (Eucommiales) diverged from an earlier ancestor.