PinkMonkey Online Study Guide-Biology
The ovule (megasporangium) contains a nucellus
is surrounded by two integuments. One diploid megaspore
mother cell is differentiated in the nucellus of a young developing
ovule. This divides by meiosis and produces a linear tetrad
of haploid megaspores. The upper three megaspores degenerate.
The basal one is functional and produces the embryo
sac in the nucellus. The embryo sac contains an 8-celled female
gametophyte differentiated into an egg apparatus, polars and
antipodals. (Figure 15.22B).
The egg apparatus consists of one egg cell and two synergids at the micropyler end in the embryo sac. The antipodals are three cells at the chalazal end. Two polar nuclei are present in the center of the embryo sac. They fuse together and form a diploid secondary nucleus.
Double Fertilization: It is similar to that described for Hibiscus (Refer to chapter section 15.4) As a result of double fertilization, the ovule develops into a seed and the ovary forms the fruit.
The fruit is a one-seeded caryopsis. It is oval or spherical, white and commonly called grain.
The seed is monocotyledonous and endospermic. The seed coat is fused with the pericarp. The embryo in the mature seed has a single cotyledon (scutellum), radicle enclosed in coleorhiza and plumule covered in coleoptile (Figure 15.22 C).
The grain on germination produces a sorghum plant..
iv) Alternation of generations: The life cycle
of sorghum shows heteromorphic alternation of generations. The sporophytic
generation is predominant. The male and female gametophytes are highly
reduced. Meiosis and fertilization are the two important
events which regulate the alternation between diploid and haploid generations.
This is evident from the graphic representation of the stages in the life
cycle of sorghum (Figure 15.23).
Click here to enlarge
Angiosperms are flowering plants. They have flowers
for reproduction and ovules encased in the ovary cavity. The pollen
grain contains highly reduced 3-celled male gametophytes. Male
gametes are non-flagellate. The gynoecium has a closed ovary,
style and stigma. Pollen grains germinate on stigma and male gametes
are carried through pollen tube (siphonogamy for double fertilization).
Endosperm in a post-fertilization triploid tissue. After fertilization,
ovules form seeds and ovary forms the fruit. Hence, seeds are
encased in the fruit (angiosperms).
Dicotyledons are the angiosperms with tap root,
reticulate venation, pentamerous flowers and dicot embryo (e.g.
Monocotyledons are angiosperms with adventitious
roots, parallel venation, trimerous flowers and monocot embryo
(e.g. Sorghum vulgare (sorghum)).
Table of Contents
15.0 - Introduction
15.1 Pteridophyta : General Account
Angiosperms : Dicotyledons
Angiosperms : Monocotyledons
Development of seed habit
Development of Flower and Fruit
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