THE CONTRIBUTION OF EMBRYOLOGY TO THE SYSTEMATICS OF THE EUPHORBIACEAE

The Euphorbiaceae are often described as a heterogeneous group with ambiguous affinities. Although their vegetative and floral characters are markedly modified in relation to the habitat and pollination mechanisms, the embryological features are comparatively more conservative, and therefore quite useful in the delimitation of as well as in understanding of relationships. The diagnostic embryological characteristics of the family are: (1) tetrasporangiate anther, 4- or 5-layered anther wall, fibrous endothecium, ephemeral middle layer(s), secretory tapetum with multinucleate cells, simultaneous cytokinesis, tetrahedral, decussate and isobilateral microspore tetrads, and 2- or 3-nucleate pollen; (2) tricarpellary, syncarpous, and trilocular ovary with one or two ovules in each locule, axile placentation, anatropous or hemianatropous, bitegmic, crassinucellate ovule, placental obturator and presence of hypostase; (3) embryo sac with ephemeral antipodal cells, nuclear endosperm and mature embryo with broad/leafy cotyledons; and (4) atbuminous seed with both integuments forrming seed coat, and outer epidermis of inner integument developing into a fibrous or sclerotic layer. Most of the 30 families included in the Euphorbiales by various authors have significant embryological differences from Euphorbiaceae. It appears more appropriate that the Euphorbiales should include only one family, the Euphorbiaceae. The closeness of the Euphorbiales with the Malvales finds support on embryological grounds. The present level of embryological information substantiates the classification of the Euphorbiaceae into five subfamilies. It seems that the Acalyphoideae diverged rather early from the Phyllanthoideae-Oldfieldioideae, and later the Crotonoideae and the Euphorbioideae evolved as independent lines from the Acalyphoideae. Since only 16 of the 50 tribes have so far been embryologically explored, comments on the tribal classification must await further probes into their reproductive biology.