Early plants

The Kingdom Plantae consists of multicellular eukaryotic plants that synthesize food by photosynthesis from carbon dioxide, water and minerals with the help of solar energy. They show unlimited growth in localized meristematic regions and have cells with cell walls made up of cellulose. They lack organs of locomotion and remain stationary. They also lack sensory and nervous systems.

Plants first appeared over 600 million years ago in the water but some later adapted to land

Algae

Algae are photosynthesising aquatic organisms. They are found in fresh and seawater. In this study, we shall consider only green algae, brown algae and red algae as under the Kingdom Plantae.

Green Algae (Chlorophyta)

Green algae are diverse, having more species than either red algae or brown algae. Most of them are freshwater dwellers while a few are marine dwellers. They range from microscopic to large forms that are over 5 meters long. They are often found living symbiotically with other organisms like the Paramecium or Hydra. They are the major source of food for aquatic plants and animals.

One of the commonest flagellated green algae is Chlamydomonas, a unicellular organism that moves by a pair of flagella. Its egg-like structure is surrounded by a cell wall composed of carbohydrate and protein and a cup-like chloroplast that contains the pyrenoid, which is the site of starch production. It has an “eye spot” within the chloroplast that can detect the direction of light and enable the organism to move toward the light. At the base of the flagella are two contractile vacuoles that discharge water from the organism.

Chlamydomonas

 Chlamydomonas reproduces by asexual and sexual reproduction. In a sexual reproduction, the nucleus divides by mitosis into two daughter cells called zygospores, each of which develops a cell wall and flagella while still attached to the parent cell. The zygospores are released when the parent cell bursts at maturity. Each flagellated zygospore then develops into a haploid Chlamydomonas. 

Life cycle of Chalamydomonas

 In sexual reproduction, Chlamydomonas divides by mitosis to produce several haploid gametes, each of which develops into a zygospore with a cell wall and a flagellum. When these are finally released from the parent cell, the gametes of the opposite sex (designated as + and -) fuse to form a diploid zygote which develops a hard wall around itself so that it is protected again severe cold and heat.

When the conditions become favourable, the zygote undergoes meiosis producing four haploid cells, each of which will develop into mature Chlamydomonas when released into the water.

Chlamydomonas-like cells are capable of forming colonial algae or aggregations of several cells. The Volvox is a spherical colonial alga that is composed of between 500 to 50000 Chlamydomonas-like cells. The various cells of Volvox seem to corporate with one another giving the Volvox a semblance of multicellularity.  Some cells are specialized for reproduction and each of these can divide asexually to form a new daughter colony.

Volvox contains green colonies which are asexually produced. During sexual reproduction (right), colonies produce sperm and egg.


Multicellular Green Algae

Spirogyra is a green, slimy alga that lives in ponds as pond “scum” It is filamentous algae consisting of rectangular cells arranged one over the other, It has a ribbon-like chloroplast which forms a spiral along the length of each cell.

Spirogyra reproduces by conjugation (like in Rhizopus). During sexual reproduction, two haploid filaments line up side by side, and form hollow conjugation tubes which extent one filament to the other. The entire contents of one of the cells, the supplying cell, stream to the receiving cell in the other filament. The supplying filament therefore acts as the motile male gamete while the receiving cell acts as the nonmotile female gamete. The fusion of the gametes produces a zygote or zygospore.

Under favourable conditions, the zygospore is released into the water, undergoes meiosis leading to the production of haploid Spirogyra filaments.

 

Reproduction in Spirogyra: Two haploid filaments are conjugating

Alternation of Generation
 Ulva is seaweed found in the sea close to the shore. It has a leafy appearance and so is called a sea lettuce.

                                                        Life cycle of Ulva
  The seaweed Ulva are multicellular green algae that show alternation of generation or a two-generation life cycle i.e. diploid saprophyte generation that produces spores by meiosis, and haploid gametophyte generation that produces gametes.
  In Ulva, both generations look exactly alike. However, the zoospores produced by the saprophyte generation develop directly into gametophyte generation while the gametes produced by the gametophyte generation fuse to form a zygote that develops into diploid saprophyte generation.

Brown Algae (Phaeophyta)

Brown algae consist of small plants, simple branched filaments to large plants of more than fifty meters long. They are found along the rocky shoreline in the temperate zones where they are pounded by the tides. They provide food and habitat for many animals. Some brown algae are eaten as food. They contain a lot of nitrogen and have therefore been used as fertiliser.

Red Algae (Division Rhodophyta)

Red algae are a group of multicellular seaweeds that sometimes attain one meter or more in length although the majority are smaller. They commonly occur in warm deep tropical waters attached to rocks and other aquatic plants. Some red algae are used as food and some are extracted into agar and used in the laboratories as growth medium for bacteria. A gelatine-like extract called carragheenin is used in chocolate making, ice cream, pudding etc.

Red algae accumulate calcium carbonate from the seawater and contribute to the formation of coral reefs.

Reproduction is by the fusion of gametes that do not have flagella and consequently sperm is transported to the female gamete by water currents.

Land Plants

The terrestrial environment was first colonised by plants about 400 mya. These small plants arose from the green algae probably the chlorophytes. To move from an aquatic environment to a land environment necessitated making major adjustments in the acquisition of water and food nourishment. Those plants that eventually made a successful transition to land life developed vascular systems for water transport, root for absorption of water and leaves for manufacturing food. Others remained very small while maintaining their aquatic environment.

Plant that made this successful transition from water to land included bryophytes (nonvascular plants) and tracheophytes (vascular plants). In both, there is the formation of specialised organs and structures; fertilised eggs develop within the tissues of the female plant; the surface of the plant is covered with a waxy cuticle that prevents loss of water by evaporation; reproduction involves alternation of generations.

There are some differences, however: tracheophytes have vascular vessels which bryophytes do not have. In addition the bryophytes lack true roots ,stems or leaves and have rhizoids for roots and leaflike structures for leaves.

Division Bryophytes

The Bryophyta (non-vascular plants)  were the first algae to establish on land about 400 mya. Although they made it to land, they lacked vascular tissue for transportation of water and food substances.

They are therefore mainly confined to damp shady places. They are quite small in size and show wide distribution worldwide but are most abundant in the tropics.They usually  a green carpet on damp soil, rocks, walls and barks of trees.

The thallus (plant body) is very small and lacks true roots, stem and leaves. It is anchored by rhizoids that also absorb water. The stem provides support and stores food, while the leaves that have stomata are capable of photosynthesising carbohydrates.

Bryophytes undergo alternation of generation life cycle involving two stages: sexual (the gametophyte) and asexual (the saprophyte). The sexes are separate consisting of a haploid male gametophyte and a separate female gametophyte. In the male plant, haploid sperm cells are produced within reproductive structures known as antheridia while the female eggs are produced within female sex organs called archegonia.

Fertilisation requires water to transport the flagellated male sperm to the female archegonium where fusion takes place. The fused cell produces a diploid zygote, which divides by mitosis into an elongated stock having a capsule or sporangium at the tip. This is the saprophyte attached to and nourished and protected by the female plant. The saprophyte forms spores within the capsule, which will be released when the capsule matures. Released spores are dispersed by wind and will germinate into branched filament called a protonema, each of which divides by mitosis to give rise to ♂and ♀gametophytes.

The following characteristics of bryophytes should be noted in relationship to higher plants and animals.

• Bryophytes have separate sexes, each of which is capable of producing gametes

• Fertilisation occurs internally inside the female body and the embryo or zygospore develops within the body of the female and is dependent upon the female for nourishment

Division Trachaeophyta

Although the bryophytes were the first plants to live on the land, they still continue to depend on moist, wet environment. By contrast, the tracheophytes are the most adapted to living on land. Among these adaptations are :

• the possession of a layer of cells that protects the reproductive organs
• multicellular embryos retained in the archegonia
• cuticle on the exposed surface that prevents water loss
• and a complete vascular system for transporting water and food substances