AN ADDITION TO THE BISPORIC EMBRYO SACS-THE DICRAEA TYPE

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1 AN ADDITION TO THE BISPORIC EMBRYO SACS-THE DICRAEA TYPE BY A. J. MUKKADA Department of Botany, University of Delhi, Delhi 6, India {Received io February 1964) SUMMARY The embryo sac in Dicraea is tetra-nucleate and follows a bisporic development. It comprises a single synergid, an egg cell and two juxtaposed antipodal cells. This justifies the establishment of a new type under the bisporic embryo sacs which may be designated as the Dicraea type. INTRODUCTION Magnus's (1913) account of the development of the embryo sac in Dicraea elongata, a member of the Podostemaceae, was not accepted by most embryologists (P. Maheshwari, 1950; S. C. Maheshwari, 1955). He reported a bisporic, tetra-nucleate embryo sac in which the first division of the functional dyad nucleus is followed by a transverse wall forming two cells. These undergo another division to form four cells which organize into a synergid, an egg and two antipodal cells. However, in other members of the family the primary chalazal nucleus of the two-nucleate embryo sac degenerates while the micropylar counterpart divides twice to form four nuclei. The mature embryo sac consists of two synergids, the egg and the upper polar nucleus along with the degenerated primary chalazal nucleus whenever present. Such a development is referred to as the reduced Alliitm type. In view of the doubts regarding Magnus's work, a re-investigation of Dicraea (two species) was undertaken and it is felt that the development of the embryo sac deserves special treatment. MATERIALS AND METHODS Materials of Dicraea stylosa and D. elongata were collected from the Punalur and Ottakkallu regions of the Kallada river, Kerala, South India. They were fixed in formalinacetic-alcohol and preserved in 70% alcohol. The usual methods of dehydration in the alcohol-xylol series followed by impregnation with paraffin wax were used. Entire ovaries were sectioned longitudinally at 7-10 pi in thickness. They were stained with Heidenhain's haematoxylin and counterstained with erythrosin or fast green. RESULTS The hypodermal archesporial cell (Fig. i) rarely two cells as in Fig. 2 functions directly as the megaspore mother cell (Fig. 3) which divides (Figs. 4 and 5) to form two unequal dyad cells. Of these, the smaller micropylar cell degenerates while the chalazal cell enlarges (Fig. 6). Its nucleus divides in an axial plane to form a two-nucleate embryo sac (Fig. 7). According to Magnus (1913) a wall is laid down in Dicraea elongata resulting 289

2 A. J. MUKKADA 10 Figs. I-I2. Dicraea stylosa. Fig. i. L.S. Young ovule with a hypodermal archesporial cell. Fig. 2. Same, with two archesporial cells. Fig. 3. Megaspore mother cell. Figs. 4 and 5. Division of megaspore mother cell; in Fig. 4 a pseudo-embryo sac is already formed. Fig. 6. L.S. O\-ule with a degenerating micropylar dyad cell and the functional chalazal dyad cell. Fig. 7. Two-nucleate embryo sac. Fig. 8. Same, both the nuclei are dividing. The formation of the secondary spindles is a noteworthy feature. Fig. 9. Same as in Fig. 8 with a different orientation of the spindles. Fig. 10. L.S. Ovule (the outer integument is not drawn) with an organized embryo sac and the pseudo-embryo sac. Fig. 11. Abnormal three-celled embryo sac. Fig. 12. Abnormal H-shaped embr\'o sac. (All figures 871).

3 A new embryo-sac type 291 in two cells. I am unable to confirm this either in D. stylosa or in D. elongata. Both the nuclei divide simultaneously. The micropylar spindle is usually in the axial plane and the chalazal spindle is at right angles to it (Fig. 8) but sometimes this too is more or less axial (Fig. 9). A rather unusual feature is the appearance of secondary spindles when the divisions reach the late anaphase (Fig. 8). There are no further divisions after this fournucleate stage and the mature embryo sac comprises a synergid, the egg and two antipodal cells (Fig. 10). The designation of the uppermost cell as the synergid and that immediately below as the egg cell may appear arbitrary but is justified since the embryo develops from the latter. The juxtaposition of the two chalazal cells and their degeneration after fertilization suggest their antipodal nature. There is no polar nucleus. Occasionally embryo sacs were observed wbere the primary chalazal nucleus had failed to divide resulting in a three-nucleate condition with a synergid, the egg and one antipodal cell (Fig. 11). A few H-shaped embryo sacs have also been observed (Fig. 12). As in all members of the family, a prominent pseudo-embryo-sac is formed directly below the female gametophyte by a dissolution of the nucellar cells (Fig. 10). It is a pre-fertilization development while in Terniola (Mukkada, 1964) it arises after fertilization. DISCUSSION The present investigation confirms the account of Magnus (1913) so far as the fate of the primary chalazal nucleus in Dicraea is concerned. It does not degenerate but divides simultaneously with the micropylar nucleus forming a tetra-nucleate embryo sac. It was suggested by Maheshwari (1950) that since the primary chalazal nucleus is very ephemeral in the Podostemaceae, Magnus might have missed some stages in Dicraea. My experience, however, is that the primary chalazal nucleus is not so ephemeral because in all the other genera I have studied ftidotristicha (Mukkada, 19626, 1963), Hydrobryum, Terniola (Mukkada, 1962^), 1964) and Zeylanidium it is readily observed. The fact that not a single instance of degeneration was observed in Dicraea may be taken as some evidence for its participation in further division. One or two instances were no doubt observed, in which the primary chalazal nucleus was smaller than the micropylar but it showed no signs of degeneration. Thus, Dicraea seems to be the only genuine instance of a tetranucleate embryo sac in the Podostemaceae. In this I support Magnus, although none of my preparations of D. stylosa or D. elongata gives any indication of a wall following the division of the functional dyad nucleus (cf. Mukkada, 1962(7). Although basically bisporic in development, the embryo sac of Dicraea differs considerably from other angiosperms. Its tetra-nucleate nature, the peculiar egg apparatus with a single synergid and egg, the absence of polar nuclei and the presence of two (occasionally only one) antipodal cells are a rare combination of characters not found in any other angiosperm not even in any other member of the Podostemaceae. It is also unique in that, unlike other angiosperms, a micropylar quartet is not formed. It differs from the reduced Allium type in that the functional dyad nucleus undergoes only two divisions as against three in the latter. These are perhaps sufficient to justify the establishment of a new type of embryo sac, the Dicraea type among other bisporic embryo sacs such as the Allium type (Maheshwari, 1950) and the Endymion type (Battaglia, 1958). This supports Palm (1915) who had already used the term Dicraea type in his review of embryo sac types. It is to be noted that in contrast to other angiosperms where the synergids are sister

4 292 A. J. MUKKADA cells, Dicraea stands out as a solitary example where the synergid and the egg are sister cells. This situation has considerable theoretical implications. It may be recalled that in explaining the homologies of the female gametophyte in angiosperms, Schiirhoff (1928; cited by Maheshwari, 1950) assumed that the egg and one of the synergids are sister cells. His theory was discarded because no instance was known where the synergid and the egg were definitely known to be sister cells. Although no attempt is made here to uphold Schurhoff's theory, the findings in Dicraea leave open the possibility of other instances of this kind being found on more critical study. The nutrition of the embryo sac is an intriguing problem associated with the embryology of the Podostemaceae. According to Magnus (1913), in normal angiosperms, the antipodal cells are concerned with the nutrition of the embryo sac and, later, of the zygote. In the Podostemaceae, Dicraea alone possesses antipodal cells. In the rest of the members there is a single nucleus which, being a member of the micropylar quartet, may rightly be regarded as the upper polar nucleus. Occasionally the degenerating primary chalazal nucleus may also persist (Mukkada, 19626, 1963, 1964). Thus, all the Podostemaceae except Dicraea have dispensed with the antipodal cells completely. ACKNOWLEDGMENT Grateful thanks are extended to Professor P. Maheshwari for guidance and criticism. REFERENCES B.ATT.\GLi.\, E. (1958). L'abolizione del tipo embriologico Scilla et la creazione dei Nuovi tipi Endymion ed Allium. Caryologia, ii, 247. M.-\GNUS, W. (1913). Die atypische Embryonalentwicklung der Podostemaceen. Flora, 105, 275. MAHESHWARI, P. (1950). An Introduction to the Embryology of Angiosperms. New York. MAHESHW.ARI, S. C. (1955). The occurrence of bisporic embryo sacs in angiosperms a critical review. Phytoinorphology, 5, 67. KK.^DA, A. J. (1962a). Some observations on the embryology' of Dicraea stylosa Wight. In: Plant Embryology: A Symposium, p. 139 ct scq. CSIR, New Delhi. KK.ADA, A. J. (1962&). Morphological and embryological studies on some Indian Podostemaceae. Ph.D. thesis, LTniversity of Delhi. MuKKAD.A, A. J. (1963). A contribution to the embryology of Indotristicha ramosissima (Wight) Van Royen. Proc. $oth Indian Science Congress (Delhi), III, 387. MUKKADA, A. J. (1964). Some aspects of the morphology, embryology and biology of Terniola zeylanica (Gardner) Tulasne. (In press.) PALM, B. (1915). Studien iiber Konstruktionstypen und Entwicklungsioege des Embryosackes der Angiospernteii. Diss., Stockholm.

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