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