Plant Kingdom

The chapter states that algae are divided into three classes depending on the type of pigment possessed and the type of stored food. So these are the basis of classification.

- Liverwort: reduction division takes place in the capsule of the sporophyte, after which haploid spores are formed.
- Moss: reduction division takes place in the capsule of the sporophyte to produce spores.
- Fern: reduction division takes place in the sporangia of the sporophyte to produce spores.
- Gymnosperm: reduction division takes place in the megaspore mother cell in the megasporangium (ovule) and in the microsporangium to form megaspores and microspores.
- Angiosperm: reduction division takes place in the microsporangium to form pollen grains and in the megaspore mother cell in the ovule to form megaspores.

Three groups of plants that bear archegonia are bryophytes, pteridophytes and gymnosperms.

Life cycle of a bryophyte:
- The main plant body is the haploid gametophyte.
- It produces antheridia and archegonia.
- The male gamete fuses with the egg in the archegonium to form a zygote.
- The zygote develops into a sporophyte, which remains attached to and dependent on the gametophyte.
- The sporophyte undergoes meiosis to form haploid spores.
- The spores germinate and grow into new gametophytes.

The ploidy in order is:
1. Protonemal cell of a moss = **haploid ($n$)
2. Primary endosperm nucleus in dicot = triploid ($3n$)
3. Leaf cell of a moss = haploid ($n$)
4. Prothallus cell of a fern = haploid ($n$)
5. Gemma cell in *Marchantia* = haploid ($n$)
6. Meristem cell of monocot = diploid ($2n$)
7. Ovum of a liverwort = haploid ($n$)
8. Zygote of a fern = diploid ($2n$)**

### Economic importance of algae
- Algae carry out a major part of carbon dioxide fixation on earth through photosynthesis.
- They increase the level of dissolved oxygen in water.
- They are primary producers in aquatic food chains.
- Many marine algae such as Porphyra, Laminaria, and Sargassum are used as food.
- Brown and red algae produce useful hydrocolloids like algin and carrageen.
- Agar from *Gelidium* and *Gracilaria* is used to grow microbes and in ice-creams and jellies.
- *Chlorella* is used as a protein-rich food supplement.

### Economic importance of gymnosperms
- Gymnosperms provide timber and other useful forest products.
- Many are grown as ornamental plants.
- Some are important sources of resin and other industrial products.
- They are also valuable ecologically because many are large trees forming forests.

From the chapter, the clearest listed uses are mainly for algae; gymnosperms are described chiefly by their characteristics, but they are economically important as useful trees and ornamentals.

Gymnosperms and angiosperms are classified separately because their seeds are not developed in the same way.

- In gymnosperms, the ovules are naked, not enclosed by an ovary wall. After fertilisation, the seeds also remain exposed and are therefore called naked seeds.
- In angiosperms, the pollen grains and ovules are found in flowers, and the seeds are enclosed in fruits after fertilisation.

So, even though both bear seeds, the presence of naked seeds in gymnosperms and fruit-enclosed seeds in angiosperms is the main reason for separate classification.

Heterospory is the production of two kinds of spores: microspores and megaspores.

Significance:
- Microspores develop into male gametophytes.
- Megaspores develop into female gametophytes.
- The female gametophyte is often retained on the parent sporophyte for some time.
- Development of the embryo within the female gametophyte is a precursor to seed habit, which is an important step in evolution.

Examples: *Selaginella* and *Salvinia*.

The terms are:
- protonema
- antheridium
- archegonium
- diplontic
- sporophyll
- isogamy

In mosses, the protonema is the first stage of the gametophyte. It develops directly from a spore and is a creeping, green, branched and frequently filamentous stage.

The antheridium is the male sex organ in bryophytes. It produces biflagellate antherozoids.