Variations in cell wall orientation and the sequence of cell divisions in the first phase of embryo development are the basis for classification of embryo types. Traditionally, six embryo types have been recognized (Johansen, 1950) and these are briefly described in Table 7.1. Within each of these embryo types, slight variations occur, leading
some authors to suggest more elaborate classification schemes. For a detailed summary on classification of embryo types see Johansen (1950) and Natesh andRau (1984). A seventh type of embryo development (Table 7.1) was reported in Paeonia L., in which the first division of the zygote and several subsequent divisions lack cell wall formation, yielding a free-nuclear proembryo (Raghavan, 1986; Johri, 2001). A large vacuole occupies the central part of the proembryo and the nuclei are peripheral. Eventually, cell wall formation takes place followed by differentiation into a typical dicot embryo. In Orchidaceae, the cell divisions are irregular and arrested so early that it is not possible to assign the embryos to a type