Friday, July 3, 2020

Plant Kingdom - Notes | Class 11 | Part 1: Classification systems, Algae


Systems of Biological classification

1. Artificial classification systems

-    Earliest systems of classification.

-    They were based on vegetative characters or superficial morphological characters such as habit, colour, number and shape of leaves, etc.

-    Linnaeus’s artificial system of classification was based on the androecium structure.


-    They separated the closely related species since they were based on a few characteristics.

-    Equal weightage to vegetative and sexual characteristics. This is not acceptable since the vegetative characters are more easily affected by environment.

2. Natural classification systems

-    These are based on natural affinities among organisms.

-    It considers external features and internal features (ultrastructure, anatomy, embryology & phytochemistry).

-    E.g. Classification for flowering plants given by George Bentham & Joseph Dalton Hooker.

3. Phylogenetic classification systems

-    It is based on evolutionary relationships among organisms.

-    This assumes that organisms in the same taxa have a common ancestor.

Other sources to resolve the problems in classification:

o Numerical Taxonomy: It is based on all observable characteristics. It is easily carried out using computers. Number & codes are assigned to all the characters and the data are processed. Thus, hundreds of characters can be equally considered.

o Cytotaxonomy: It is based on cytological information like chromosome number, structure, behaviour etc.

o Chemotaxonomy: It uses chemical constituents of plants.


-    Algae are simple, thalloid, autotrophic, chlorophyll-bearing and aquatic (fresh water & marine) organisms.

-    They also occur in moist stones, soils and wood.

-    Some occur in association with fungi (lichen) and animals (e.g., on sloth bear).

-    The form and size of algae is highly variable.

o Microscopic unicellular forms: E.g. Chlamydomonas.

o Colonial forms: E.g. Volvox.

o Filamentous forms: E.g. Ulothrix and Spirogyra.


-    Vegetative reproduction: By fragmentation. Each fragment develops into a thallus.

-    Asexual reproduction: By the production of spores. E.g. zoospores (most common). They are flagellated (motile) and on germination gives rise to new plants.

-    Sexual reproduction: Through fusion of two gametes. It is many types:

o  Isogamous: Fusion of gametes similar in size. They may be flagellated (e.g. Ulothrix) or non-flagellated (non-motile, e.g. Spirogyra).

o  Anisogamous: Fusion of two gametes dissimilar in size. E.g. Some species of Eudorina.

o  Oogamous: Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete. E.g. Volvox, Fucus.

Benefits of algae:

  Through photosynthesis, they fix half of the total CO2 on earth and increase the level of dissolved oxygen.

  They are primary producers and the basis of the food cycles of all aquatic animals.

  Many marine algae (70 species) are used as food. E.g. Porphyra, Laminaria and Sargassum.

  Agar (from Gelidium & Gracilaria) is used to grow microbes and in ice-creams and jellies.

  Some marine brown & red algae produce hydrocolloids (water holding substances). E.g. algin (brown algae) and carrageen (red algae). These are used commercially.

  Protein-rich unicellular algae like Chlorella & Spirullina are used as food supplements by space travellers.

Algae include 3 classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.

1. Chlorophyceae (green algae)

-    Unicellular, colonial or filamentous.

-    They are usually grass green due to the pigments chlorophyll a and b in chloroplasts.

-    The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

-    Most of them have one or more pyrenoids (storage bodies) located in the chloroplasts. Pyrenoids contain protein besides starch.

-    Some algae store food as oil droplets.

-    They have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.

-    E.g. Chlamydomonas, Volvox, Ulothrix, Spirogyra & Chara.


o Vegetative reproduction: By fragmentation or by formation of different types of spores.

o Asexual reproduction: By flagellated zoospores produced in zoosporangia.

o Sexual reproduction: Isogamous, anisogamous or oogamous.

2. Phaeophyceae (brown algae)

-    They are mostly marine forms.

-    They show great variation in size & form. They range from simple branched, filamentous forms (E.g. Ectocarpus) to profusely branched forms (e.g. kelps- 100 m in height).

-    They have chlorophyll a, c, carotenoids & xanthophylls.

-    They vary in colour from olive green to brown depending upon the amount of a xanthophyll pigment, fucoxanthin.

-    Food is stored as complex carbohydrates (laminarin or mannitol).

-    The vegetative cells have a cellulosic wall covered by a gelatinous coating of algin.

-    Protoplast contains plastids, central vacuole and nucleus.

-    Plant body is attached to substratum by a holdfast, and has a stalk (stipe) and leaf like photosynthetic organ (frond).

-    E.g. Ectocarpus, Dictyota, Laminaria, Sargassum & Fucus.


o Vegetative reproduction: By fragmentation.

o Asexual reproduction: By pear-shaped biflagellate zoospores (have 2 unequal laterally attached flagella).

o Sexual reproduction: Isogamous, anisogamous or oogamous. Union of gametes occurs in water or within the oogonium (oogamous species). Gametes are pear-shaped (pyriform) bearing 2 laterally attached flagella.

3. Rhodophyceae (red algae)

-    They have a red pigment, r-phycoerythrin.

-    Majority are marine especially in the warmer areas.

-    They occur in both well-lighted regions close to the surface of water and at great depths in oceans where relatively little light penetrates.

-    The red thalli of most of the red algae are multicellular.

-    Some of them have complex body organisation.

-    The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.

-    E.g. Polysiphonia, Porphyra, Gracilaria and Gelidium.


o Vegetative reproduction: By fragmentation.

o Asexual reproduction: By non-motile spores.

o Sexual reproduction: Oogamous. By non-motile gametes. It has complex post fertilisation developments.


Chlorophyceae (Green algae)

Phaeophyceae (brown algae)

Rhodophyceae (Red algae)

Major pigments

Chlorophyll a, b

Chlorophyll a, c, Fucoxanthin

Chlorophyll a, d, Phycoerythrin

Stored food


Mannitol, laminarin

Floridean Starch

Cell wall


Cellulose and algin


Flagellar number & position of insertion

2-8, equal, apical

2, unequal, lateral



Fresh water, salt water & brackish water

Fresh water (rare), salt water & brackish water

Fresh water (some), salt water (most) & brackish water

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