EMBRYOTAXONOMY

            A. INTRODUCTION

            Embryotaxonomy is a branch of modern approach to taxonomy or synthetic taxonomy. It deals with the study of embryology, i.e., study of micro and megasporogenesis, gametophyte development, fertilization, and development of endosperm, embryo and seed coats in relation to taxonomic study.

            B. BASIC EMBRYOLOGICAL CHARACTERS USEFUL IN TAXONOMY

Some embryological characters which have proved to be of special importance in taxonomic considerations include –

(1) Presence and type of anther tapetum, (2) Number and arrangement of anther loculi, (3) Type of anther endothecium, (4) Quadripartition of microspore mother cell, (5) Mature pollen grain, (6) Development, structure, position, vasculation and orientation of ovule, (7) Origin of sporogenous tissue in ovule, (8) Microsporogenesis and the development of embryo sac, (9) Presence of aril, (10) Form of the embryo sac, (11) Fertilization, (12) Type of embryo, (13)  Type of embryogeny, (14) Endosperm formation, (15) Type of haustorium formation, (16) Seed coat, (17) Cotyledons, etc.

C. ROLE OF EMBRYOLOGY IN SOLVING TAXONOMIC PROBLEMS

A lot of literature has been published on the role of embryology in solving taxonomic problems during the last 60 years. A few examples are given below –

1. Angiosperms are universally divided into dicotyledons and monocotyledons. This primary classification of angiosperms is based on the major embryological character, i.e., number of cotyledons. In dicot seed, the number of cotyledon is two, while in monocots it is one.

2. The embryological characters such as trinucleate pollen, bitegmic crassinucellate ovules which are campylotropous or amphitropous, seed with peripheral embryo, and perisperm with little or no endosperm, are the characters which are found only in caryophyllales, more widely known as Centrospermae (Cronquist, 1968).

3. The monocotyledonous order Helobiae treated as a sub-class in some recent systems of classification, is characterized by the presence of helobial type of endosperm.

4. The distinguishing embryological character of the members of the order Orchidales is the presence of undifferentiated embryo and a very little or no endosperm.

5. The members of Podostemaceae family are recognized because of the formation of pseudo-embryo sac which is formed by the disintegration of nucellar cells below the embryo sac. The presence of paired pollen grains, tenuinucleate ovules and prominent suspensor haustoria, and absence of antipodals and triple fusion are other characteristics of the family.

6. The family Onagraceae is recognized by the presence of Onagrad type of embryo-sac. Such an embryo sac is derived from the micropylar megaspore of the tetrad. The lower three chalazal megaspore do not disintegrate. In this type of embryo sac, antipodals are absent.

7. Paeonia has been treated by most of the taxonomists as a member of the monogeneric tribe Paeonieae of the family Ranunculaceae. But Paeonia differs from Ranunculaceae in its chromosome number, vascular anatomy and floral anatomy as well as in embryological details.

8. Because of the presence of naked ovule and pollen chamber, Exocarpus was removed from Santalaceae of angiosperms and was treated as a member of the family Exocarpaceae near Taxaceae in gymnosperms. But because of the presence of typical angiospermic flower, polygonum type of embryo sac, cellular endosperm and some other embryological characters, it confirms that Exocarpus belongs to the family Santalaceae of angiosperms, and not with the gymnosperms.

9. Majority of the taxonomists treated Trapa as a genus of Onagraceae.While other considered it to be a genus of Hydrocaryaceae. But its embryological details (Polygonum type of embryo sac, absence of endosperm, well developed suspensor haustorium, extreme reduced one cotyledon, etc.) suggests that Trapa should be treated under an independent family Trapaceae.

10. The presence of Polygonum type of embryo sac in Butomus and the Allium type of embryo sac in other genera of the family Butomaceae, suggests that only Butomus should be retained in Butomaceae. The other genera of this family should be transferred either to Alismataceae or to Limnocharitaceae.

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