A large oval cell in the nu-cellus of the ovule of flowering plants within which fertilization occurs. Initially it is the megaspore mother cell, which divides meiotically, normally forming four megaspores. Typically three of these abort and the remaining one divides mitotically to give the haploid cells of the embryo sac. This form of development is termed monosporic. Usually there are three mitotic divisions to give an eight-nucleate embryo sac. The nuclei are arranged as shown in the illustration. However in some plants, e.g. Oenothera, there are only two mitotic divisions giving a four-nucleate embryo sac (lacking the three antipodal cells and one of the polar nuclei). In bisporic development, two megaspores contribute to the formation of the mature embryo sac. For example, in Allium the cells of the embryo sac develop from one of the two cells formed after the first division of meiosis (the other product of the reduction division aborting at this stage). Three divisions of this cell give rise firstly to two megaspores (a dyad) and then to an eight-nucleate embryo sac.
In some plants all four megaspores may continue development (tetrasporic development). In such cases there may be one mitotic division to give an eight-nucleate embryo sac, e.g. Adoxa, or two mitotic divisions, giving a sixteen-nucle-ate embryo sac, e.g. Drusa. The arrangement of cells in embryo sacs derived by tetrasporic development may show a variety of forms.
The mature embryo sac represents the female gametophyte, the egg cell being the gamete. See also double fertilization.
Emerson effect (enhancement effect) The observation (made by Robert Emerson in 1957) that photosynthesis, which proceeds very slowly using light of 700 nm wavelength, can be greatly increased when chloroplasts are also illuminated with light of shorter wavelength (650 nm). This was a surprising observation as it was then thought that light absorbed by the chlorophylls and other pigments was all passed on to a small percentage of chlorophyll a molecules (the energy trap) absorbing at 700 nm. This and later work indicated a second energy trap absorbing at 680 nm. See Photosystems I and II