Psilopsida, Lycopsida, Sphenopsida And Pteropsida.

Psilopsida, Lycopsida, Sphenopsida And Pteropsida.

PSILOPSIDA (PSILOPHYTA)

This group includes vascular plants without true leaves and roots. The plant body is composed of only root. It shows regular alternation of generation.

Sporophyte generation

The sporophyte generation of psilopsida shows following characteristics:

1. Psilopsida have rootless and leafless sporophyte.

2. The stem is divided into an underground part rhizome and an aerial part.

3. The stem is branched dichotomously.

4. The rhizome develops rhizoids. Both rhizome and rhizoid perform the function of root.

5. The aerial braches are green, leafless. They have small veinless outgrowths. These branches carry out photosynthesis.

6. The reproductive organs of sporophyte are sporangia. These sporangia develop at tips of branches or at the lateral side of branches.

7. Internal structure of stem is simple. They have narrow, central, and solid vascular tissues. Pith is absent in them. Their cortex is wide.

Gametophyte generation

Their gametophyte is thalloid. It is colourless and undersround. Its cells contain a fungus. This fungus provides food to the gametophyte and gametophyte provides it protection. Such beneficial symbiotic relationship among the two members (fungus and plants) is called mycorrhizal association. Classification

Psilopsida is the earliest group of vascular plants. Most of the representative of this group has become extinct, for example, Rhynia, Horneophyton, Psilophyton, and Cooksonia.

LYCOPSIDA (Club mosses/spike mosses)

1. The strobili of lycopsids are club or spike shaped. Their leaves area small resembling to the leaves of mosses. So they are also called club or spike mosses.

2. The plant body of Lycopsida is sporophyte. It is differentiated into roots, stem and true leaves.

3. The leaves are small and single veined. They are also called microphylls. The arrangement of leaves is spiral* or opposite.

4. There are certain reproductive leaves called sporophylls. The sporangia develop singly on the upper side of the sporophylls. The sporophylls may arrange to form strobili or cones.

5. There are two types of sporophytes in lycopsids:

➤ Homosporous sporophytes: The spõrophytes of some Lycopods have one kind of sporangia like Lycopodium. They produce single type of spore. Such sporophytes are called homosporous. This condition is called homospory.

➤ Heterosporous sporophytes: Some lycopsids - have two types of sporangia microsporangia and megasporangia as in Selaginella. They produce two types of spores. Such sporophyte is called heterosporous. This condition is called heterospory.

6. The gametophyte of Lycopsida is mainly underground.

7. Example: Selaginella is a heterosporous plant and it resembles the seed producing plants (spermatophytes).

SPHENOPSIDA (Horsetail)

1. The sporophyte of Sphenopsida is differentiated into roots, stem and leaves.

2. The leaves may be expanded or scale like. The leaves are always arranged in whorls.

3. Complete plant body is composed of a large number of joints. So these plants are also called arthrophytes.

4. Main stem is not smooth. It has a large number of ridges and furrows. It has nodes and internodes. Each node has whorl of branches.

5. The sporangia are borne on structures called sporangiophores. The sporangia aggregate on sporangiophores to form cones or strobilus. Each sporangiophore has a slender stalk and an expanded disc at its free end. The sporangia are present on the underside of the disc.

6. Their gametophyte is thalloid. It grows on clayey soil or on mud.

PTEROPSIDA

It is most advanced sub -division. Pteropsida is divided into three classes:

(i) Class Filicineae

(ii) Class Gymnospermae

(iii) Class Angiospermae

CLASS FILICINEAE (ferns)

1. The Filicineae or ferns are mostly shade and moisture loving plants. A very few plants of this group grow under dry condition. They grow on hills and plains. Some plants are epiphytic and grow on the bark of trees. Although ferns are worldwide in distribution but they are especially abundant in the tropics (cold areas).

2. The class Filicineae contains foliar sporangia. Their sporangia are attached to frond or leaves.

3. Their leaves are called fronds. The fronds are coiled when they are immature and young. This pattern of development is called circinate vernation. It is an important character of this group.

4. They vary greatly in size,

5. Examples: Dryopteris, Pteridium, Adiantum and Pteris etc.

GYMNOSPERMAE (Non-flowering plants)

Naked seeded non-flowering plants are called gymnosperms. They are one of the most successful groups of seed plants. They have world wide distribution. They constitute about 1/3 of the world forests. They show following characters:

1. The term Gymnospermae means 'naked seeded (Gymno = naked and spermae = seed). The ovules in these plants are not enclosed. The ovules are borne on the exposed surface of fertile leaves. These fertile leaves are called megasporophyll.

2. The gymnosperms are heterosporous plants.

3. The show regular heteromorphic alternation of generation.

➤ Sporophyte: They have independent and dominant sporophyte. It produces two types of spores, microspores and megaspores. Microspores are developed on microsporophyll and megaspores are developed on megasporophyll. The megasporophylls cover ovule. These sporophylls are not folded or joined at the margins to form an ovary. Therefore, the seeds remain naked on megasporophyll.

➤ Gametophyte: The gametophyte is less conspicuous and dependent. The female gametophyte is permanently retained within the ovule.

4. The important genera of Gymnospermae are:

➤ Cycas: sago-palm

➤ Pinus: pines

➤ Picea: Hemlock

➤ Cedrus: deodar

➤ Ginkgo

CLASS ANGIOSPERMAE

The flowering plants with their seeds enclosed in fruits are called angiosperms. The term angiosperm means "enclosed seeded" (angi = close and sperm = seed). Their fertile leaves bear ovules. The fertile leaves are folded and joined to from ovaries. The 'ovary is changed into a fruit after fertilization. Fruit contains seed. There are 235,000 species of angiosperms out of 360,000 known species of plants. Angiosperms are heterosporous plants. These are most successful plants of the earth. Theses plants produce flower, fruits and seeds.