THE CONTRIBUTION OFEMBYROLOGY TO THESYSTEMATICS OF THEEUPHORBIACEAE'R. N. KapiP and A. K. Bhatnagar2ABSTRACT The Euphorbiaceae are often described as a heterogeneous group with ambiguous affinities. Although their vegetativeand floral characters are markedly modified in relation to the habitat and pollination mechanisms, the embryologicalfeatures are comparatively more conservative, and therefore quite useful in the delimitation of as well as in understandingof relationships. The diagnostic embryological characteristics of the family are: (1) tetrasporangiate anther, 4. orS-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, anatropousor hemianatropous, bitegmic, crassinucellate ovule, placental obturator and presence of hypostase; (3) embryo sacwith ephemeral antipodal cells, nuclear endosperm and mature embryo with broad/leafy cotyledons; and (4) albuminousseed with both integuments forming seed coat, and outer epidermis of inner integument developing into a fibrous orsclerotic layer. Most of the 30 families included in the Euphorbiales by various authors have significant embryologicaldifferences 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. Thepresent 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 theCrotonoideae and the Euphorbioideae evolved as independent lines from the Acalyphoideae. Since only 16 of the 50trbes have so far been embryologically explored, comments on the tribal classification must await further probes intotheir reproductive biology. The Euphorbiaceae represent a large group oflowering plants distributed in almost ail parts ofthe world, except the arctic regions. The familyincludes 300 genera and 8000 species (Webster,1987a) predominantly distributed in the tropicaland subtropical areas of both the New and OldWorlds. Because of the great variability in vege-tative and floral characters, the group has beendscribed as heterogeneous and highly diversified(Webster, 1967; Corner, 1976). It is being increasingly realized that a naturalor phylogenetic classification, be it of the angio-Sperms as a whole, or of a class, order, or family,cahnot be based solely on morphological charac-ts. The external features are often drasticallyR <died in relation to environmental stress andai to refleet true phyletic trends. Embryology has,therefore, acquired an important place, along withotr disciplines such as cytology, anatomy, pal-ynlogy, and chemotaxonomy, as a dependable tooli tematics. Embryological features are es-Ssignificant in situations where exomor-Pology either leads to inconclusive correlations orna even delude due to convergence (Kapil &Bhatnagar, 1980). Wagenitz (1975) rightly em-phasized that "for elucidation of the systematicposition of groups with simple flowers embryologicaland phytochemical characters are especially valu-able, as classical characters of flower may be tooseriously altered by reduction." The embryologicalcharacters have also proved useful in: (i) deter-mining the limits of a taxon, especially at the levelof a family or order; (ii) ascertaining affinities whentwo or more possibilities are suspected; and (iii)evaluating a scheme of classification. In the Euphorbiaceae, the sporophytic charac-ters exhibit tremendous diversity. They help inrecognizing natural groups down to the level ofspecies and even below. On the other hand, thehighly reduced gametophytic generations displaymuch less variability. They are quite uniform atthe generic level and above. They are more con-vincing not only in circumscribing the family, butalso in judging its affliations and in classifying itinto subfamilies and tribes. Considering the large size, the Euphorbiaceaeare embryologically poorly known. Only 16 tribes(and about 30 genera), out of 50 tribes recognizedSWehak Michael J. Huft, Geoffrey A. Levin, and two anonymous reviewers for commenta and suggestio.".rtaent of Botany, University of Delhi, Delhi.110 007, India. ANN. MIssouR BOT. GARD. 81: 145-159. 1994.
BioStor
