Recurrent Type

In this type, euspory (seen in normal cases) is replaced by aneuspory (Diplospory) because of irregular meiosis. When the spore mother ceIl functions directly as the embryo sac initial it is termed gonial apospory and when one or more of the somatic cells of the nucellus or chalaza act as embryo sac initial it is called somatic apospory.

Aneuspory (Diplospory)

Includes cases in which during spore formation the meiotic process is irregular. Depending upon the extent of irregularity in meiosis in various plants, aneuspory is further categorized into various types.

• Datura type - the two unreduced nuclei undergo two mitotic divisions to produce an 8-nucleate embryo sac.
• Taraxacum type - first meiotic division ends in a restitution nucleus. Meiosis II produces unreduced cells. Ultimately an 8-nucleate embryo sac is formed. The egg directly forms the embryo.
• Ixeris type - it is like Taraxacum, but a single binucleate gynophore is formed. Two divisions lead to the formation of an 8-nucleate embryo sac.
• Allium type - a premeiotic-endomitotic doubling makes meiotic prophase to start with double chromosome number. The result is an unreduced embryo sac.

Posted Date: 1/23/2013 5:55:47 AM | Location : United States

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