The megasporangium together with its protective coats the integuments is called ovule. It is attached to the placenta on the minor wall of the ovary by a stalk called funiculus. An ovule ready for fertilization consists of nucellar tissue enveloped almost completely by one or two integuments leaving a small opening at the alrical end. This opening called micropyle, is the main passage for the entry of pollen tube into the ovule
The basal region of the ovule where funiculus in attached is called chalaza in the nucleus is present the female gametophytes, also called embryo sac
INTEGUMENTS:
The protective coverings of nucleus are called integuments. Mostly an ovule has either one or two integuments ovules with one integument are called unitegmic those with two integuments are known as bitegmic
The sympetalae predominantly show unitegmic condition. Bitegmic ovules occurs in polypetalae, moncots in some members of Olacaceae, Crinum, Viscum, Loranthus, Santalum, Balanophora etc. The ovules that lacks integuments are calld ategmic
Ontogenetically, the ovule arises as a small mound of homogenous tissue on the placenta in the ovary. Integuments arises close to the base of this tissue which forms the nucleus in the nature ovule
Except for Euphorbiaceae where the inner integuments is initiated sub dermally in all others it is dermal in origin . The outer integuments is initiated either dermally or sub dermally . Although the integuments initiated later they grow faster than the nucleus soon surround it almost completely except in the region of the micropyle
The unitegmic condition of ovules is considered to the derived from fusion of the two integuments as in some Myrtaceae, or by suppression of one integuments.
Various degrees of fusion among parts is a common feature of ovule; the two integuments in a bitegmic ovule may be fused along their length or the inner integuments may be fused with the nucleus upto various lengths. In anatropous ovules very often the outer integuments on the side of the funiculus is almost indistinguishable from the funiculus because of their congenital fusion.
In some taxa especially belonging to the family Cactaceae, a prominent air space is present between the two integuments in these chalazal region. This feature is also shown by Basia, Petragonia tetragonoides and Tricanthema acquatius.
Occurence of stomata on the outer integument has been reported in Cleome, Canna, Nelumbium, Isomeris. In Gossypium stomata differentiate into chalazal region of the outer integument two days before anthesis. Such stomata are suggested to serve in respiration rather than in a transpiration or in photosynthesis.
In addition to stomata abundant chlorophyll is present in the integuments of Hymenocallis, Amaryllis, Gladiolus, Lilium, Moringa
Endothelium:
In most plants belonging to sympetalae with unitegumic ,tenuinucellate ovules, the nucellus degenerates at as early stage of ovule development and the inner most layer of the integument becomes specialised to perform the nutritive function from the embryo sac. This specialised tissue present around the embryo sac is called endothelium. So far the occurrence of endothelium has been observed in 65 families of dicots. The endothelium is usually single layered, in Compositae it may become multi-layered, ten to twelve layered endothelium is known in sunflower.
An interesting feature of the endothelial cell is the development of adventitive embryos. Maheshwari Devi and Pullaiah have reported that in Melampodiun divaricatum sometimes the endothecial cells look like egg and undergo divisions forming structure resembling zygotic embryos. Such embryos lack suspensor and in this respect duffer from the zygotic embryos.
In Begoniacea, Droseraceae, Elamtinaceac, Papilionaceace the persistent nucellar cells from endothelium like tissue because of different origin has been called false endothelium
MICROPHYLE:
Depending upon the presence or absence of integuments the microphyle may or may not organized. In the bitegemic ovules the microphyle is generally formed by either both the integuments only the inner integuments. Only rarely does the outer integuments alone constitute the microphyle.
When both the integument are involved the passage formed by outer integument is called Exostone and that by the inner integument is called Endostone. In post fertilisation stages a plug is formed that occludes the microphyle. The plug probably has a protective role against dessication and pathogen invasion.
OBTURATOR
Any ocular structure associated with directing the growth of pollen tube toward the microphyle is generally referred to as obturator
Obturator exhibit great variation in their origin, morphology, anatomy and extent of development. They may originate from placenta or funicullus or both.
The most common type of obturator is one formed by local swelling if the funiculus(Anacardiaceae, Labiatae). In Crinum the funiculus simply becomes knee shaped function as obturator. In Tetragonia tetragonoides the obturator comprising glandular epidermal hairs, arises from both side if the long funiculus.
Placental obturator occurs in the Euplurbiaceace, Cuscutaceae. In Aegle some epidermal cells of funiculus as well as placenta elongate into richly cytoplasmic multicellular hairs reaching upto microphyle.
The pollen tube grows along the obturator. The cells of the obturator produce a surface exudate and provide nutrition and mechanical and chemical guidance to the pollen tube
NUCELLUS:
It represents the wall of megasporangium. Each ovule had only one nucellus. As an abnormality however twin nucelli may occur in a common fold of integuments this had been observed in Aegle marmelos Hydrocleis etc..
The archesporium differentiates immediately below the nucellar epidermis. In sympetalae the acchesporial cell directly function as the megaspore mother cell so that the sporogenous cell is also hypodermal.
Such ovules where the sporogenous cell is hypodermal the nucellar tissue around it remains single layered are called tenuinucellate.
In some other families the hypodermal archesporial cell divides transversely cutting an outer parietal cell an inner sporogenous cell. The parietal cell may either remain undivided or undergo periclinal and anticlinal divisions so that the sporogenous cells becomes embedded in the massive nucleus. The sporogenous cell may also become embedded in the nucellar tissue by divisions in the nucellae epidermis.
All such ovules where the sporogenous cells become sub hypodermal either due to the formation of parietal cells or due to division in the nucellar epidermis or both called crassinucellate..
Davis (1966) has however suggested that only these ovules should be referred to as crassinucellate where the sporogenous cells become sub hypodermal due to the occurrence of parietal cells..
She has proposed the term pseudo crassinucellate for all those ovules where divisions in the nucellar epidermis are a responsible for the sub hypodermal nature of the sporogenous cell..
According to Davis of the 314 families for which informal is available 179 families show crassinucellate ovules, 105 families bear tenuinucellate ovules and 11 families posses pseudo-crassinuccelate ovules.
Generally the nucellus remains within the confines of the inner integuments rarely however it may project into the microphyte or beyond if forming a nucellar beak. Nucellar beak has been reported from Euphorbiaaceae, cucurbitaceae, Nyctaginaceae, polygonaceae, Salicaceae of cells may store starch protein crystals
The nucleus is mostly consumed by the developing embryosac or endosperm. In some plants it persists in the mature seed as a nutritive tissue. The persistent nucelllus is called perisperm.
There is other extreme where the nucellar tissue breaks down precocious consequently a large cavity called pseudo embryo sac is formed around the embryo sac. This feature is unique to the family Podostemaceae. In absence of endosperm in the Podostemaceae, the pseudo embryosac which contains cytoplasm for nuclei, nourishes the developing embryo.
HYPOTASE:
It refers to a group of cells present below the embryosac and above the vascular supply to the funiculus. They become thick walled due to lignification and are poor in cytoplasmic contents.
Hypotase occurs in many families such as Amaryllludaceae, liliaceae, zingiberaceae, Euphorbiaceae, Theaceae, Umbellifareaceae, in the horanthacea a Hypotase is present below the archesporium.
EPITASE:
Refers to a group of cells present above the embryosac. This tissue persists as a hood over the apex of the embryosac for a long time in Costus and Castalia. It forms a cup like structure of cutirized cells and is distinguishable even during advance stages of embryo development.
Types of Ovules
Orthotropous
1. The ovule is straight, without any curvature
2. Micropyle, chalaza, funicle and embryo sac lie in a straight line.
3. Ex – Polygonum, Piper
Anatropous
1. Body of ovule becomes completely inverted to 1800.
2. The micropyle lies close to funicle
3. Micropyle, Embryo sac and chalaza lie on the same line.
4. Ex – Helianthus, Ricinus
Campylotropous
1. The body of the ovule is placed at right angles to the funiculus.
2. The body of ovule bends in such a way that micropyle comes towards funiculus.
3. Micropyle and chalaza do not lie on the same straight line.
4. Ex – Pisum, Mustard
Dic
No comments:
Post a Comment