Deviations from the Polygonum type of megagametophyte development are based on the following morphological criteria:

1. The number of megaspores or megaspore nuclei that participate in the formation of the embryo sac.
2. The total number of divisions that take place during the formation of the megaspore and gametophyte.
3. The number and arrangement of the nuclei and their ploidy level in the mature embryo sac.

Based on the number of megaspores, embryo sacs can be divided into three types: monosporic, bisporic, and tetrasporic (Web Figure 21.3.A). In the monosporic, or Polygonum-type embryo sac, meiosis of the diploid megaspore mother cell in the nucellus produces four haploid megaspores. Three of the megaspores, usually those at the micropylar end of the nucellus, subsequently undergo programmed cell death, leaving only one functional megaspore. In bisporic embryo sacs, meiosis produces only two megaspores, each containing two haploid nuclei, due to the absence of cytokinesis and cell plate formation following the second meiotic division. The megaspore nearest the micropyle then undergoes programmed cell death, leaving a single functional megaspore with two haploid nuclei. In tetrasporic embryo sacs, cell plates fail to form after both meiotic divisions, resulting in a single four-nucleate megaspore. All three patterns give rise to a single functional megaspore that contains either one (monosporic), two (bisporic), or four (tetrasporic) haploid nuclei. Note that the nuclei of the bisporic and tetrasporic embryo sacs are not genetically identical as they are in monosporic embryo sacs, because they arise from two or four different meiotic products.

Web Figure 21.3.A Three main types of megagametophyte development: monosporic, bisporic, and tetrasporic.