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PinkMonkey Online Study Guide-Biology

Perfect (bisexual) spikelet (Fig. 15.21C). It is sessile and comparatively broader and larger. The axis of the spikelet is short and bears two flowers (florets). The lower flower is sterile and non-functional (i.e., without sex organs). The upper flower is perfect (i.e. functional and bisexual). Each flower has two bracts; one called the lemma and the other, the palea.

Structure of a perfect flower:

Perianth is represented by two colorless, small, broad and hairy perianth members called lodicules.

Androecium has three free stamens. Anthers are bilobed, bithecous and versatile.

Gynoecium is monocarpellary. Ovary is superior, with one ovule in a basal placentation. There two long and free styles. Stigmas are short and bushy.

The pedicelled staminate spikelet also has florets. The lower one is sterile while the upper one is staminate. It has three stamens and two lodicules, but a gynoecium is absent. Each floret has a lemma and a palea. The spikelet is enclosed in two glumes-outer and inner.

iii) Reproduction: Sorghum is heterosporous.

The micropores or pollen grains are produced in the anther locules (microsporangia). Each anther locule produces many diploid pollen mother cells. These divide by meiosis and produce tetrads of haploid pollen grains. Young pollen grains are uninucleate.

The male gametophyte is highly reduced in size and is produced within the microspore (pollen) wall. A fully formed male gametophyte consists of only three cells namely a vegetative cell and two male gametes. The mature pollen contains a 3-celled gametophyte at the time of pollination. The pollen wall has two layers, intine and exine. Exine is thick, smooth and has one germ pore (Fig. 15.22A).

Figure 15.22 (A) A mature pollen grain with 3-celled male gametophyte. (B) Longitudinal section of an ovary showing structure of ovule
and embryo sac and passage of pollen tube during fertilization. (C) Seed (grain) structure

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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