Category : 12th Class
(1) Development of embryo (Embryogeny) : The zygote after a period of rest develops into embryo. The process of development of mature embryo from diploid zygote is called embryogenesis.
(i) In dicotyledons : The normal type of dicot embryo development has been studied in Shepherd's purse (Capsella bursapastoris) family Cruciferae. This is called as crucifer or onagrad type of embryo development. This development of embryo is endoscopic i.e., apex is downward or towards inside. The first division of zygote is transverse which produces a basal cell (cb) towards the micropyle and a terminal cell (ca) towards chalaza. The basal cell divides by transverse division and the terminal cell by a longitudinal division, so 4 celled T-shaped proembryo is produced. The two basal cells divide by transverse division and form 6-10 celled suspensor. The upper most cell of the suspensor is vasicular cell and lowest cell is called hypophysis which forms radicle and root cap.
The two apical cells first divide by longitudinal division (at right angle to first one) and then by transverse and periclinal division. So sixteen celled globular embryo is produced. Due to differentiation of cotyledons globular embryo becomes heart shaped.
Mature embryo in dicots consists of two lateral cotyledons, terminal plumule or stem tip and radicle or root tip.
(ii) In monocotyledons : The normal type of monocot embryo development has been studied in Sagittaria sagittaefolia. The early development of dicot and monocot embryos is similar upto globular stage. Later on differentiation starts. Suspensor is single celled and vascular. There is only one terminal cotyledon called scutellum (shield shaped). In grasses the second cotyledon is reduced called epiblast.
The basal cell (cb) divides by a transverse wall into two cells – ci and m. The cell ci divides once again to form n and n’ cells. Of these n’ is the outermost which develops into suspensor. The cell n forms parts of root cap the cell m contributes to the remaining part of root cap and a part of the radicle.
The terminal cell (ca) divides by two vertical walls, at right angles to one another. This results in the formation of a quadrant (q). Cells of the quadrant divide periclinally differentiating into the peripheral cells and the inner group of cells. The repeated divisions in both peripheral and central group of cells results in the formation of two regions –l and l’. Region l produces the lower part of cotyledon while upper part of cotyledon, hypocotyl and plumule are formed by l’ region.
(2) Polyembryony : Occurrence of more than two embryo in the seed is known as polyembryony. It was discovered by A.V. Leeuwenhock (1719) in Citrus. It may be :
(i) Cleavage polyembryony : Due to cleavage of zygote or proembryo into two or more embryos and each split part develops into an embryo. This type of polyembryony is common in gymnosperms than in angiosperms. Erythronium americanum, Nymphaea advena, Crotalaria, etc., are some of the angiosperms showing cleavage polyembryony.
(ii) Simple polyembryony : Due to presence of more than one embryo sac and so oospore or egg. e.g., Brassica.
(iii) Mixed polyembryony : More than one pollen tube entering an ovule and fertilizing synergids (as in Argemone maxicana) and antipodal cell (as in Ulmus americana).
(iv) Adventive polyembryony : Diploid nucellus or integument cells form embryos e.g., Citrus, Opuntia, Mangifera.
If extra embryos develop from same embryo sac, it is called true polyembryony and if embryos develop elsewhere it is called false polyembryony. In Balanophora, an extra embryo develops from endosperm.
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