Kingdom Plantae

Kingdom Plantae

Life began on earth about 3.5 thousand million years ago. About 420 million years ago, the first organism appeared on land. These were the earliest plants, which are autotrophic eukaryotes adapted for life on land. The only other autotrophic eukaryotes are algae, which are specialized for life in water.

All these plants have some common features that include:

• All members are multicellular eukaryotes and are able to prepare their food by photosynthesis.
• The cell is extremely covered with the cellulosic cell wall.
• They lack locomotory organs.
• The majority of members are autotrophs. Some are heterotrophs such as insectivores or symbiotic.
• Starch is the reserve food material.

Algae:

The name ‘algae’ has been derived from the Latin word ‘alga’ meaning ‘seaweed’. This group contains near about 18,000 genera with 29,000 species.

Algae are a very large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. They may be present in colonial or filamentous forms as well. The filament may be branched (Cladophora) or unbranched (Spirogyra). They may be free floating or attached to the substratum by holdfast or chapter.

The vegetative plant body is gametophyte and haploid. It is thalloid and lacks differentiation into roots, stem, and leaves.

Algae show great variation in their habitat. Some algae like Oedogonium in habitat still water, while Cladophora primarily grows in running water. Red algae like Batrachospermum are found in freshwater. Few lives in marine water, too.

In the modern study, the branch of science that deals with the study of algae is known as ‘phycology or algology’

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Classification of algae

Algae possess chlorophyll. Many of them particularly red and brown algae are not green in colour because of the presence of their pigments that mask the green pigment. Based on the photosynthetic pigments, the algae are grouped into three classes: Chlorophyll (green algae), Phaeophyceae (brown algae) and Rhodophyceae (red algae).

1. Chlorophyceae (Green algae)

• This division comprises more than 7000 species of algae.
• They live in a wide variety of habitats from salt to freshwater.
• Most of the green algae live in freshwater, either as part of the plankton or attached to submerged rocks and twigs. Some are marine species. Some green algae are components of lichens as phycobiont partner. Some species lack chlorophylls and live as parasites upon other plants. Few green algae Zoochlorella are associated with animals as shells of mollusks, sponges etc.
• They exhibit an extraordinary variety in form and shape. They may be unicellular motile (Chlamydomonas), unicellular non-motile (Chlorella), colonial motile (Volvox), colonial nonmotile (Hydrodictyon), filamentous unbranched (Ulothrix), filamentous branched (Cladophora), umbrella-like (Acetabularia), etc.
• The cells contain four types of photosynthetic pigments including Chlorophyll a, Chlorophyll-b, Xanthophyll and carotene enclosed in chloroplast. The green colour is due to the presence of excess chlorophyll.
• The cell is externally covered with the cell wall made up of cellulose and peptone.
• Protoplast possesses central vacuole, cell organelles such as mitochondria, Golgi body, endoplasmic reticulum, etc.
• Many green algae bear pyrenoids that store starch.
• They consist of the well-defined nucleus.
• They reproduce by asexual and sexual methods.
• Asexual reproduction occurs by zoospores, a planospores, hypnospores, akinetes, tetraspores, etc.
• Sexual reproduction takes place by gamete formation. It may be of the isogamous, anisogamous or Oogamous type.
• After syngamy, the diploid zygote is formed, which represent the diploid generation.

1. Phaeophyceae (Brown algae)

• There are about 1500-2000 species in this division of algae.
• Members of this division are multi-cellular. No unicellular members have been reported yet in this group.
• These algae consist of chlorophyll a and chlorophyll-c, and fucoxanthin.
• Laminaria is attached to the rocks in the sea due to the presence of holdfast. Sargassum is free floating algae abundant in warmer seas.
• Cell wall consists of the additional covering of phacocolloids (alginic acid) that protects algae from drying and freezing, and mechanical stress.
• Asexual reproduction is commonly found. It may occur by fragmentation.
• In many forms of brown algae, unilocular sporangia are formed that produces biflagellate zoospores. A multicellular ellipsoidal structure called Pleurilocular sporangia may form in some other forms of brown algae like Ectocarpus. Dictyota produces non-motile tetraspores.
• Sexual reproduction is variable It may be of isogamous or Oogamous.
• Rhodophyceae (Red algae)
• These are one of the oldest groups of eukaryotic algae.
• They are one of the oldest group, with about 5,000-6,000 species. Most of them are multicellular, marine algae, including many notable seaweeds.
• They may range from unicellular to half a meter in length.
• They contain phycoerythrin and phycocyanin along with pigment chlorophyll and carotenoid.
• The reserve food material is usually Floridean starch and galactoside Floridoside.
• The algae lack flagellated cells.
• The cell wall is made up of Polysulphate esters along with pectic materials. Cellulose is also present.
• Reproduction occurs by the asexual and sexual method.
• Asexual reproduction occurs by mean of non-motile spore.
• Sexual reproduction is highly specialized. Male gamete is called spermatia, which is non-motile.
• Female sex organ is called Carpogonium.

Spirogyra

Kingdom: Plantae

Division: Chlorophyta

Class: Chlorophyceae

Order: Conjugales

Family: Zygnemaceae

Genus: Spirogyra (spiro-coiled; gyra-curved)

Spirogyra is a very common free-floating and chlorophyllous freshwater alga found in great abundance in pools, ponds, lakes, etc. Some species like Spirogyra rhizopus remains attached to the substratum with the help of rhizoids like basal cells of the filament i.e., chapter.

Due to the presence of a mucilaginous substance around filaments, Spirogyra forms a green silk mass on the surface of stagnant water. Hence, these are called ‘pond scum or water silk or pond silk’.

Structure

The plant body is gametophytic thallus. The thallus is filamentous and unbranched with many cells placed end to end. All the cells of the filament are cylindrical and alike in their structure. In attached forms of Spirogyra, the filaments develop basal cell i.e., Hapteron or holdfast which help in the attachment to the substratum.

Each cell of Spirogyra is covered externally with the double layered wall, made up of outer pectin and an inner cellulose layers. The pectic substance of outer layer dissolves in water to form the slimy sheath (third layer). The adjacent cells of the filament are separated by transverse septa which may be the plane, replicate or sub-replicate, etc.

Each cell is uninucleate, with the nucleus usually situated at the center of the cell and connected by cytoplasmic strands to the dense cytoplasm of the peripheral region i.e., primordial utricle. The cell consists of a big central vacuole. In Spirogyra, the chloroplast is spiral and ribbon-like. It may be serrated or smooth at the margins. The number of chloroplasts ranges from 1-14 in different species. Many proteinous bodies i.e. pyrenoids are found at the regular interval in the chloroplast, which stores starch.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.