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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.

  • Pollination is anemophilous, i.e. by wind.

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).

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SUMMARY: ANGIOSPERMS

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. Hibiscus rosa-sinesis).

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
15.2 Gymnosperms
15.3 Angiosperms : Dicotyledons
15.4 Angiosperms : Monocotyledons
15.5 Vascularization
15.6 Development of seed habit
15.7 Development of Flower and Fruit

Chapter 16





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