[255 ] BY P. MAHESHWARI AND ASHRAFUL. (With 15 Figures in the Text)

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1 [255 ] THE EMBRYO SAC OF CHRYSANTHEMUM '. PARTHENIUM L. (BERNH.)* BY P. MAHESHWARI AND ASHRAFUL HAQUE (With 15 Figures in the Text) Palm, in 1916, gave the first detailed account of the development of the embryo sac in Chrysanthemum par theniiim {= Pyrethrum parthenifoliitmvar. aiireum). According to him the megaspore mother cell undergoes the usual meiotic divisions to form the four megaspore nuclei which are not separated from one another by walls. Vacuoles soon arise between the nuclei, and as the embryo sac elongates they gradually increase in size. Meanwhile, the nuclei take up a more or less linear arrangement and by two further divisions, which take place synchronously, a sixteen-nucleate embryo sac arises which organizes to form a three-celled egg apparatus, two polar nuclei, and eight antipodal cells of which the basal is four-nucleate. Recently Fagerlind (1941) has made a fresh study of the embryo sac of this species based on collections from two separate plants. Since these were found to differ from one another in some respects, they are described separately. In the first specimen, collected from the ' Bergianischen Garten', Stockholm, the four megaspore nuclei were found to occupy the most variable positions with respect to one another. Frequently the lower three nuclei were seen in close contact with one another, thereby giving the impression that they were on the point of fusion. Only one definite instance of fusion was observed, however, and in this only the two basal nuclei had fused. Shortly afterwards the embryo sac undergoes much elongation so that the nuclei, which were previously adjacent to one another, now become separated from one another by vacuoles. At this stage the micropylar megaspore nucleus usually lies alone in the upper end of the embryo sac, although sometimes two nuclei were seen in this position. In size also the micropylar nucleus exceeds all the remaining nuclei which show a progressive decrease in volume in proceeding from the micropylar to the chalazal end. All the nuclei now divide simultaneously, resulting in four pairs of nuclei of which the two basal nuclei are the smallest. These usually fail to take part in the next division so that the last stage of the embryo sac shows fourteen instead of the sixteen reported by Palm. Sometimes fewer than fourteen nuclei were seen, but this was due to a subsequent degeneration of some of the original nuclei of the chalazal end. In the fully organized embryo sac the upper antipodal cells are uninucleate but the basal cell contains several nuclei which fuse at a later stage to form one nucleus. In the second specimen, collected by Dr I. Holmgren, the megaspore mother cell as well as the developing embryo sacs and their nuclei were found to be of larger size than the corresponding stages of the first plant. Further, the basal megaspore nucleus degenerates soon after its formation. The remaining three nuclei divide to produce six and then * Paper submitted in commemoration of the 70th birthday (22 June 1948) of Prof. G. Tischler (University of Kiel). 17-2

2 256 p. MAHESHWARI AND ASHRAFUL HAQUE Figs

3 The embryo sac of Chrysanthemum parthenium L. {Bernh) 257 twelve nuclei. Here again, in the mature embryo sac, the basal antipodal cell was observed to have more than one nucleus while the other antipodal cells are uninucleate. In the summer of 1946, during his tour in the U.S.A., the author (P.M.) saw some flowering specimens of this species in the Brooklyn Botanic Gardens, New York, and collected some inflorescences with a view to checking upon the observations of Palm and Fagerlind and seeing if the diflerences could be reconciled in any way.* The fixations were made on the spot in formalin-acetic-alcohol and the subsequent processes of dehydration, embedding and sectioning completed in India. Since our observations difler in some respects from those of Palm as well as Fagerlind, they are briefly recorded below. The ovule is unitegmic and tenuinucellate. The hypodermal, archesporial cell functions directly as the megaspore mother cell and is often mounted on two or three axial cells of the nucellus (Fig. i). The two meiotic divisions proceed normally and result in the formation of two (Fig. 2) and then four (Fig. 3) nuclei. The arrangement of the four megaspore nuclei is somewhat variable. In some cases they lie in a single row (Fig. 3); in others they show a more or less cross-wise arrangement (Figs. 4-6); and occasionally they are seen in pairs, the micropylar pair being separated by a vacuole from the chalazal pair (Fig. 8). Fig. 9 shows an abnormally long embryo sac with the four nuclei lying in a linear row and separated from one another by large vacuoles. No appreciable diflerence could be noticed between the nuclei, although occasionally there is a slight diminution in their size from the micropylar to the chalazal end. There are no nuclear fusions, although in certain cases the two central megaspore nuclei were found to lie in close contact with each other (Figs. 5. 6). The eight nuclei of the next stage lie in four pairs, each pair being separated from the next by a vacuole (Fig. 10). Preparatory to the last division, the micropylar pair of nuclei is further removed from the remaining three pairs by an increase in the volume of the vacuole between the first and the second pair of nuclei (Fig. 11). The next stage (Fig. 12) shows sixteen nuclei, four at the micropylar end and twelve at the chalazal. These organize to form a three-celled egg apparatus, two polar nuclei and eleven antipodal cells, * Grateful thanks are due to the Director, Brooklyn Botanic Gardens, for permission to collect the specimens, and to Mr J. Durkin and Miss E. Clarke for help in fixing the material. Explanation of Figs All x 800 Fig. I. L.s. nucellus showing megaspore mother cell. Eig. 2. Two nuclei formed after the first meiotic division. Eig. 3. Four-nucleate stage; megaspore nuclei arranged in a linear fashion. Eig. 4. Same, showing cross-wise arrangement of nuclei. Eig. 5. Same, the two central nuclei lie close to each other. - Fig. 6. Same, showing beginning of vacuolation. Eig. 7. Older stage showing more advanced vacuolation. The two nuclei at the micropylar end lie at right angles to the chalazal nuclei. Eig. 8. The four nuclei lie in two pairs separated by a large vacuole. Eig. 9, The four nuclei are arranged in a linear fashion separated from each other by large vacuoles. Eig. 10. Eight-nucleate stage, showing four pairs of nuclei. Eig. II. Same, showing more pronounced elongation of the sac. Eig. 12. Embryo sac showing sixteen free nuclei. Fig. 13. Fully developed embryo sac showing two synergids, an egg, two polar nuclei and eleven antipodal cells. (The nucleolus of the upper polar nucleus has slipped out of the nuclear membrane owing to accidental pressure on the cover-glass.) Fig. 14. Chalazal end of the sac, where the second antipodal cell from the base is binucleate. Fig. 15. Chalazal end of the sac, where the third antipodal cell from the base is binucleate.

4 258 p. MAHESHWARI AND ASHRAFUL HAQUE all of which are usually uninucleate (Fig. 13). In contrast with the observations of Palm and Fagerlind, the basal antipodal cell never showed more than a single nucleus. Nor was it found to differ appreciably from the other antipodal cells in size. But sometimes one or the other of the remaining antipodal cells may be binucleate. In Fig. 14 the second cell from the base has two nuclei and in Fig. 15 the third cell shows the same condition. Eventually, all of the antipodals degenerate forming a dark slender streak which is entirely absorbed during later development. Since the main object of the present investigation was to study the development of the embryo sac, other observations are only incidental. It was noted, however, that as in other Compositae the nucellar epidermis disorganizes during the development of the embryo sac and the innermost layer of the integument becomes differentiated as the endothelium. A peculiar feature is that most of the cells of the anther tapetum degenerate in situ, and only a few protrude inside into the anther loculus. A true periplasmodium was never seen in our material. It is concluded that the embryo sac of Chrysanthemum parthenium is tetrasporic and arises by two divisions of the megaspore nuclei, but there are differences with regard to the exact number of nuclei in the mature embryo sac and the organization of the antipodal cells. These differences may be related to environmental conditions or there may be different races of this species which behave somewhat differently from one another. Only further studies can settle this point. REFERENCES FAGERLIND, F. (1941). Die Embryosackentwicklung bei Tanacetum vulgare und einigen Chrysanthemum- Arten. Svensk. Bot. Tidskr. 35, PALM, B. (1915). Studien iiber Konstruktionstypeii und Entwicklungswege des Embryosackes der Angiospemien. Diss. Stockholm. {Received 20 August 1948)

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