NEET Biology Sexual Reproduction in Flowering Plant / फूलों के पौधों में यौन प्रजनन Embryo and Endosperm

Embryo

Embryo

(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 Shephered 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.

Endosperm

Endosperm is the nutritive tissue for the developing embryo and also the seedling. In angiosperms, the endosperm develops from triploid (3n) primary endosperm nucleus which is formed as a result of vegetative fertilization, triple fusion or fusion of a male gamete with secondary nucleus of the central cell.

(1) Types of endosperm: On the basis of development, endosperm are of three types:

(i) Nuclear endosperm: In the nuclear type of endosperm development, the primary endosperm nucleus divides by repeated mitotic free nuclear divisions without the formation of walls. It results in the formation of a large number of free nuclei in the central cell of the embryo sac. A big central vacuole develops in the embryo sac pushing all the nuclei to the peripheral cytoplasm. Finally cell wall formation takes place from the periphery of the embryo sac towards the centre leading to the formation of cellular endosperm tissue. In Coconut, the endosperm is multicellular in the outer part and free nuclear in the centre. Nuclear endosperm is the most common type of endosperm and mostly found in polypetalae. e.g. Cotton, Zeamays, Capsella etc.

(ii) Cellular endosperm: In the cellular type of endosperm development, the first nuclear division of the primary endosperm nucleus is immediately followed by the wall formation. The first division results in the formation of two equal sized chambers : chalazal and micropylar chambers. The subsequent divisions are followed by regular cell wall formation. This type of endosperm formation is common in gamopetalae. e.g. Petunia, Datura.

(iii) Helobial endosperm : In the helobial type of endosperm development, the endosperm is intermediate between cellular and nuclear types. The division of primary endosperm nucleus is followed by wall formation and as a result two chambers : micropylar and chalazal chambers, are formed. Generally the chalazal cell does not divide further and function as haustorium. Nucleus of the large micropylar cell divides by repeated free nuclear divisions and further development takes place in the same way as the nuclear endosperm. Helobial type of endosperm development is prevalent in monocotyledons. e.g. Erumurus.

(2) Some terms related to endosperm

(i) Ruminate endosperm : Mature endosperm with irregularity and unevenness in its surface is called ruminate endosperm. Rumination is caused by the activity of seed coat or by the endosperm itself. It is found in about 32 families of angiosperm. e.g. Annonaceae, Palmae, Myristicaceae, etc.

(ii) Mosaic endosperm : In some cases, the tissue of endosperm is not homogeneous but there are patches of different colours. Such type of endosperm is called mosaic endosperm and was observed by Webber (1990) in Zea mays. In maize endosperm, red and white patches appear irregularly distributed. In Petunia and Tomato, endosperm shows two types of tissues – some consisting of diploid cells and some triploid cells. These two types of cells intermix to form mosiac.

(iii) Xenia : The effect of pollen on endosperm is called xenia. This term was given by Focke (1881). e.g. Maize.

(iv) Metaxenia : The effect of pollen of somatic tissue lying outside the endosperm is known as metaxenia. Metaxenia term given by the swingle (1928). e.g. Datepalm.