Megasporangium and Female Gametophyte

Johnson (1902) reported that in P. medium and in P. aduncum and also in two species of Hekeria (the related genus), there was only one primary archesporial cell and the embryosac development was that of the "Lilium type". The megaspore mother cell was found to form no tetrad of megaspores but as a result of three free nuclear divisions followed by cell formation gave rise to a 7-celled, 8-nucleate embryosac (currently called the Adoxa type of embryosac). Later Johnson (1910) studied the embryosac of P. betle var. monoicum and found the same type of embryosac development in this species also. Fischer (1914) and Palm (1915) investigated the embryosac development in P. tuberculatum and P. subpeltatum respectively and their results agreed with those of Johnson. But later workers (Schnarf 1936,

Figure 2.16 L.S. of ovary showing different stages of ovule development. 1, 2. Development of ovule (x73); 3, 4, 5. L.S. of bisexual flower showing ovule and stamens (x37) st—stamen; std—staminode; uh—uniseriate hair.

Maheswary 1937) suspected the "Adoxa" type of embryosac and based on published literature, they came to the conclusion that the embryosac development followed the "Fritillaria type". This was later supported by Swamy (1944) in P. betle, Joshi (1944) in P. longum and Kanta (1962) in P. nigrum.

Maugini (1950) studied the female gametophyte in P. geniculatum and P. ungiculatum and interpreted the development to be "Euphorbia dulcis type" characterized by triploidy in its chalazal half and that the 8-nucleated gametophyte originated from the secondary tetra-nucleated gametophyte. Later Kanta (1962) studied the development of female gametophyte in black pepper in detail on which the following discussion is mainly based. The ovary in black pepper is unilocular and contains a single ovule, and the placentation is basal (Fig. 2.16). The ovule is tritegmic, crassinucellate and orthotropous. The ovular primordium arises at the base of the ovary when the ovary wall is still developing. Further development of the ovary and ovule proceeds simultaneously. The integument and archesporium differentiate more or less at the same time. By the time the embryosac reaches the 4-nucleate stage the micropyle appears to be fully developed. The integuments are generally 3-5 layered while the tips are made up of 7-9 layers and appears swollen.

Usually a single hypodermal cell gets differentiated as an archesporial cell. This undergoes a periclinal division giving rise to the parietal cell and the megaspore mother cell. The sporogeneous cell enlarges and function as the megaspore mother cell. The nucleus then undergoes meiotic division resulting in a 4-nucleate embryosac, where the cells are arranged either in linear fashion or in a cross-wise manner. Gradually the central nuclei move towards the enlarged end, leaving only one nucleus at the micropylar end (1+3 arrangement). Then all the nuclei simultaneously undergo meiotic divisions. During this division the spindles of the three nuclei situated at the chalazal end fuse and a secondary 4-nucleate stage results in which the two micropylar nuclei are haploid and the two chalazal nuclei are triploid. The four nuclei undergo another mitotic division giving rise to the 8-nucleate embryosac. Thus the embroysac is the "Fritillaria" type. The mature embryosac shows the normal 3+2+3 arrangement of nuclei; i.e. two synergids and the egg nucleus are at the micropylar end, 2 nuclei (secondary nucleus) in the middle and the three antipodals in the chalazal end. Kanta (1961) also observed that in about 50 per cent of the ovules the embroyosac degenerates and the ovule collapses. The ovules may abort before or after fertilization. The unfertilized ovule may abort at the primary 4-nucleate stage, but more frequently at or after the 8-nucleate stage (Kanta 1961). The process of development of the female gametophyte is shown in Fig. 2.17.

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