Post-fertilization changes within the Ovule-digieduco

Post-fertilization changes within the Ovule :Post-fertilization changes within the Ovule is a series of changes take place within the ovule after fertilization resulting finally in the formation of seed. The zygote i.e. oospore (2n) gives rise to (a) embryo, cells of which are diploid (2n):(b) the triploid (3n) nucleus or so-called endosperm nucleus gives rise to endosperm, the cells of which are triploid (3n) ; (c) the ovule together with embryo and endosperm as a whole gives rise to seed which remains covered by the seed coat developed from integument(s).

DEVELOPMENT OF EMBRYO (EMBRYOGENY)-
In angiosperm ,the zygote develops into embryo simultaneously with the development of the endosperm. In most cases, the first division of the zygote is followed by the laying down of a transverse wall but in a few cases, e.g. in Piperad type such division of the zygote takes place by the formation of a more or less vertical wall. Of the two cells thus formed, the one which lies towards the centre of the embryo sac is called the terminal cell and the one nearer the micropyle is called the basal cell. Theterminal cell, next may divide longitudinally or transversely ; the basal cell usually undergoes transverse division forming the suspensor, sometimes the basal cell does not divide and becomes hypertrophied forming a large vesicular structure.

(i) Typical dicotyledonous type i.e. Crucifer type-Typical dicotyledonous type of embryo development is found in Capsella bursa-pastoris of the family Cruciferae (Brassicaceae). Zygote divides first transversely into a basal cell cb and a terminal i.e. apical cell ca. Next cb divides transversely into ci and cm cells, and ca divides longitudinally resulting in the formation of an inverted T (i.e. ‘_I_')-shaped proembryo composed of four cells. Now each of the two terminal cells divides again longitudinally at right angle to the first longitudinal division forming a quadrant (4-celled) structure. Quadrant cells then become octant (8-celled structure) by a transverse division. The lower four cells of the octant give rise rise to stem tip and cotyledons while the upper four cells to the hypocotyl and the core of the radicle. Now all the eight cells of the octant divide periclinally (i.e. parallel to the outer surface) forming outer and inner layers.

In the mean time cells of and cm of the four-celled proembryo divide transversely forming a row of several (6-10) suspensor cells-the uppermost cell of which becomes swollen, and vesicular (v) which probably serves a haustorial function ; the lowermost cell (h) functions as the hypophysis which later divides further and gives rise to epidermis and cortex of the radicle and root cap. The long suspensor pushes the embryo deep into the embryo sac. Further development of the embryo proper also takes place at two points in the cells of the lower tier forming more or less a cordate structure. By transverse cell divisions, cotyledons and hypocotyl elongate-the growing cotyledons become horse shoe-shaped structure due to horse shoelike curvature of the ovule. As the embryo matures, suspensor cells gradually wither.

(ii) Typical monocotyledonous type of embryo development is seen in Luzula forsteri of the family Juncaceae. Zygote divides into a basal cb and a terminal ca cells. Then the terminal cell ca divides longitudinally into two cells while the basal cell cb divides transversely into ci and m. cells. Next two cells of ca divides vertically at right angle to the first and give rise to quadrant q, cell m divides longitudinally into two cells while of divides transversely into n and n ’cells. By successive divisions the cells of quadrant become differentiated into I and l’ portions-l gives rise to lower part of cotyledon while l upper part of the cotyledon, hypocotyl and plumule; m and n give rise to root cap and n' to the short suspensor (o and p cells)

(b) ENDOSPERM FORMATION-Endosperm is the main source of the food for the embryo, hence it is very important. In angiosperms, endosperm tissue is triploid (3n) as it is the product of triple fusion [as a result of a fusion of two polar nuclei (n + n) and one of the male gametes (n)] involving double fertilization. In angiosperms, except the two families viz. Orchidaceae and Podostemonaceae, there are three general types of endosperm formation, e.g. (a) nuclear type, (b) cellular type and (c) helobial type.

Development of both the zygote and endosperm takes place simultaneously in angiosperms.

(1) Nuclear type-The endosperm nucleus divides and redivides forming numerous free nuclei . These free nuclei are normally unaccompanied by walls, at least in the beginning. The nuclei are pushed towards the periphery of the embryo sac, as a result a vacuole is formed in the centre. Sometimes nuclei are aggregated towards the micropylar and chalazal ends ; free nuclei remain suspended peripherally in the cytoplasm lining the wall of the embryo sac. In later stages wall formation occurs by the laying down of cell plates which begin from periphery of the sac towards the centre or from the apex of the sac towards the base. This type of endosperm formation is known as nuclear type. While the embryo and endosperm formation are taking place within the embryo sac, synergids and antipodal cells are disorganised. Nuclear type of endosperm formation is found in many plants like species of Primula, Mangifera, Malva, Juglans, Asclepias, Calotropis, etc.

(2) Cellular type-In this type, the division of the primary endosperm nucleus is immediately followed by the formation of walls within the sac, so that the endospem: becomes cellular from the beginning. The first wall is generally transverse but may Vertical or oblique sometimes. Each cell contains one or more than one nucleus. Cellular type of endosperm formation is noted in species of Adoxa, Peperomia, Centranthus, etc and in many other members of Annonaceae, Gentianaceae, Boraginaceae, Aristolochiaceae etc.

(3) Helobial type-This type is intermediate between the nuclear and the cellular types. Helobial type of endosperm formation is noted in species of Eremurus (Liliaceae) Scheuchzeria (Scheuchzeriaceae), Vallisneria (Hydrocharitaceae) etc. Here after the first division of the primary endosperm nucleus, transverse wall is formed between the two daughter nuclei so that the embryo sac is divided into two, i.e. micropylar and chalazal chambers. Further divisions are free nuclear type and may take place in both the chambers. But usually the main body of the endosperm is formed by the micropylar chamber only.

(c) FORMATION OF SEED-The ovule i.e. megasporangium develops into seed after the complete development of the embryo. The integuments develop into seed coat-the outer integument into testa while inner integument into tegmen. Stalk of the seed deveIops from the funicle ; hilum, micropyle, raphe etc. of the ovule develop into the corresponding parts of the seed.


If the endosperm and nucellus are completely absorbed by the developing embryo, the seed is called exalbuminous ; if endosperm is retained partly, then the seed is called albuminous. Sometimes nucellus is also retained as perisperm outside the endosperm- then the seed is called perispermic.

Besides testa and tegmen, a third fleshy covering or outgrowth known as aril or arillus is often developed in some seeds after fertilization e.g. Litchi chinensis, Nephelium longana, etc. of Sapindaceae. Aril is always found more loosely arranged than the true integuments, and generally extends only partially over them ; it grows from the outer cell layers of the outer integument or from the base (chalaza or the distal part of the funicle) of the ovule.

It is not infrequent to find outgrowths from various parts of the testa (outer integument) which may be confused with the aril-such an outgrowth formed from the micropylar rim of the testa is called a false aril or arillode ; this type of outgrowth is noted in nutmeg (Myristica fragrans, Myristicaceae) which is usually described as an aril-it originates. however, from both the hilum and micropyle ; it forms a scarlet covering to the seed and forms the valuable spice ‘mace’ of commerce.

Outgrowths of an irregular character are often developed from outer parts of the testa-where the outgrowths are confined to crest as long as the raphe, they are called strophioles, and where restricted near the base ( Viola tricolor, Violaceae) or apex (Ricinus communis, Euphorbiaceae) of the seed are called caruncle,-these structures are always developed subsequently to fertilization, and accordingly are not found on the ovule.