Embryology in relation to taxonomy
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The document provides a detailed overview of taxonomic characters in various plant families, emphasizing embryological and pollen characteristics that assist in classification. It discusses specific families like Cyperaceae, Centrospermales, Loranthaceae, Onagraceae, and Trapaceae, highlighting unique features such as microspore development and embryo-sacs. The significance of pollen attributes in taxonomy is also outlined, noting how variations in pollen structure can aid in identifying plant families and species.
Embryology in relation to taxonomy
- 1. Dr. T. A. Gitte, Head, Dept. of Botany, Vaidyanath College, Parli-Vaijnath
- 2. According to Maheshwari (1964) and Bhojwani and Bhatnagar (1978), the characters of taxonomic value in delimiting plant groups include the: (a) Anther; (b) Quadripartition of the microspore mother cell; (c) Development and organisation of the pollen-grain, (d) Development and structure of the ovule; (e) Origin and extent of the sporogenous tissue in the ovule; (f) Megasporogenesis and development of the embryosac; (g) Form and organisation of the mature embryosac; (h) Fertilisation; (i) Endosperm; (j) Embryo and (k) Seed-coat.
- 3. Cyperaceae: In the family Cyperaceae, while all four microspore nuclei are produced after meiosis, three of them are cut off on one side of the pollen grain and only the fourth develops to form the generative cell and then the male gametes. All genera and species of Cyperaceae studied in Europe and Japan (and at Delhi by Mr. C.K. Shah) show this character and it is possible to identify a member of this family just as definitely by a microscopic study of its anthers as by other floral characters. Further, the simultaneous type of microspore formation and the functioning of all the four microspores in the Juncaceae indicate that it is this family from which the Cyperaceae have probably been derived.
- 4. Centrospermales: The Cactaceae agrees with the rest of the Centrospermales in having the following embryological characters: (a) Glandular anther tapetum whose cells become two-to four-nucleate; (b) Microspore-mother cells is which two meiotic divisions are succeeded by a simultaneous quadripartition into the microspores; (c) Trinucleate pollen-grains; (d) Campylotropous ovules with strongly curved funiculi and massive nucellic;
- 5. (e) A hypodermal archesporial cell which cuts of a wall cell; (f) A micropyle formed by the swollen tips of the inner integument which protrude out and approach the functions; (g) Formation of a nucellar cap originating from, periclinal division of cells of the nucellar epidermis; (h) Functioning of the chalazal megaspore of the tetrad: (i) Formation of a monosporic eight-nucleate embryosac; (j) Functioning of the perisperm as the main storage region; (k) Disappearance of most of the endosperm in the mature seed generally leaving merely a single-layered cap over the radicle.
- 6. Loranthaceae: The studies of Johri and associates (1957) on the Loranthaceae show that the Loranthoideae is embryologically different from the Viscoideae as regards mode of development of embryosac, endosperm, embryo and in the location of the viscid zone of the fruit and that the subfamilies should be raised to the status of families.
- 7. Onagraceae and Trapaceae: A monosporic tetranucleate embryo-sac is characteristic of all members of the Onagraceae and is not found in any other family of angiosperms. The genus Trapa having an eight- nucleate embryo-sac, which was once placed in the Onagraceae, has since been removed and assigned to a new family Trapaceae. Manasi Ram’s (1956) work on Trapa bispinosa fully confirms this view. Earlier, Eames (1953) expressed the view that on anatomical evidence also Trapa does not belong to the Onagraceae and is not even closely related to it. Table I presents the embryological differences between the families Onagraceae and Trapaceae.
- 9. Pollen characters: Polynology is the science of pollen and spores and its applications. It is derived from the Greek word palynein meaning to scatter. The significance of pollen attributes in taxonomy has been realised during the last three decades. The outer wall of pollen-grains is endowed with unique structural traits which are broadly categorised in order of their importance in phylogeny into the apertures, exine ornamentation, exine strata, shape and size.
- 10. According to Bailey and Nast (1943), “there are families of dicotyledons in which the pollen is of very considerable taxonomic significance not only in the differentiation of subfamilies and tribes but also of genera and species”. A few examples will illustrate the statement. The Caryophyllales is recognised by centrospermous type of pollen with a spinulose and punctate-perforate tectum. The Malvaceae and the Compositae contain typically spinulose exine, the Plumbaginaceae verrucate pollen and the Gramineae smooth, sulcate ones.
- 11. Number of nuclei in pollen: The number of nuclei in the pollen at the time of dispersal has been used by taxonomists. The angiosperm pollen is either binucleate or trinucleate according to the precocity of division of the generative nucleus. The binucleate condition is considered as more primitive than the trinucleate. In the Centrospermae, the pollen is uniformly trinucleate. The monocot (Liliaceae) is binucleate, the apetalous and polypetalous dicots are binucleate and gamopetalous members trinucleate.