Describe structure of ovule (Megasporangium)
An ovule (megasporangium)
may arise from the placenta at the base of the ovary or on the inner surface of the
ovary. It is borne on a distinct stalk, the funicle which is attached to the
body of the ovule at a point called the hilum. The central and somewhat conical
part of the ovule is the nucellus. It consists of a mass of parenchymatous cells.
It represents the metasporangium proper and is invested by two-layered
integuments in monocotyledons and primitive dicotyledons (Archicliamydeae) but
by single-layered integument in the higher dicotyledonous families
(metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the same
family. In many cases, single integument has resulted due to the fusion of two
separate primordials or by elimination of one of the two integuments. The third integument in the form or aril is found in plants like Asphodehus and
Trianthema, while Ricinus and several other plants of the family,
Euphorbiaceae, show an integumentary proliferation called the caruncle at the
micropylar end.
A very peculiar
condition is reported in opuntia where the funicle becomes very long and
completely surrounds the ovule, thus looking like a third integument. In
parasites like Santalum (Sandalwood) and Loranthus, there is no integument.
The integuments arise from the chalaza which constitutes the basal part of the nucellus.
They grow upward in close contact with the nucellus and enclose the latter
completely, leaving only a small opening, the micropyle at the apex. An embryo
sac is embedded in the nucellus towards the micropylar end.
The embryo sac
as will be seen later bears the embryo and is the most important part of the
ovule. In a mature ovule, the embryo sac bears towards the micropylar end three
protoplasts constituting the egg apparatus of those one is the egg or oosphere
while the other two are the synergids or help cells. At the chalazal end are
three antipodal cells enclosed in the cell wall. In the center of the embryo sac
lies the secondary nucleus.
The vascular supply
of the ovule is supplied by a vascular strand that runs through the funcile
and extends upwards, but rarely beyond the chalaza.
In some cases as
in Gmynosperms, the vascular bundles enter the integuments or even the nucellus
also. Sometimes the integuments are green due to the presence of chlorophyll
e.g. Amaryllis and bear stomata e.g. Nerium and Gossypium.
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