In this article we will discuss about:- 1. Description of Phaeophyceae 2. Characteristics of Phaeophyceae 3. Classification.
Description of Phaeophyceae (Brown Algae):
It is a large group of algae consisting of 240 genera and over 1,500 species out of which 32 genera and 93 species are reported from India. They are commonly known as brown algae, due to the presence of a golden brown xanthophyll pigment, fucoxanthin (C40H54O6) in the chromatophores.
About 99.7% members are marine and a few grow in fresh water. The fresh water members are Pleurocladia, Heribaudiella, Pseudobodanella, Lithoderma and Sphacelaria. Members like Pleurocladia lacustris grow both in fresh water and marine habitats.
Important Characteristics of Phaeophyceae (Brown Algae):
The important characteristics of the class Phaeophyceae are given below:
1. Plant body is immobile, multicellular and highly differentiated both externally and internally. [Unicellular, colonial (motile and non-motile) and unbranched filamentous forms are completely absent).
2. They range from simple microscopic heterotrichous filament (Ectocarpus) to largest alga (Macrocystis pyrifera), which attains a length of 60-90 meters. (The largest forms are known as kelps or rockweeds. Lessonia davicans reachs a length of 4 meters and looks like a miniature tree. Nereocystis luelkeana, the bladder kelp which attains a length of 25-30 meters. Postelsia palmae- formis appears like a palm tree and commonly known as Sea Palm) (Fig. 3.109).
3. Commonly the plant body is differentiated into hold fast, a short or elongated stipe and an expanded blade. The blade performs photosynthesis and bears reproductive structures. Many species remain afloat by having air bladders.
4. The photosynthetic pigments include chlorophyll a, chlorophyll c, β-carotene and xanthophylls like lutein, fucoxanthin, flavoxanthin and violaxanthin. The fucoxanthin is however present sufficiently which partially mask the chlorophyll and carotenoid, thereby giving the characteristic brown colouration.
5. The growth of the plant body may be apical (Fucales, Dictyotales), intercalary (Laminariales) or trichothallic (Ectocarpales).
6. The cell wall is differentiated into outer and inner layers. The outer mucilaginous layer has fucinic and alginic acid, but the inner layer is mainly cellulosic. [The alginic acid is used to manufacture artificial silk and adhesive, obtained commercially from Sargassum, Laminaria etc].
7. The cells usually have many small vesicles and white granules. The granules are called fucosan vesicles.
8. Pyrenoides are usually absent, but, if present, is of single stalk type.
9. Motile structures (zoospores and gametes) have two laterally inserted unequal flagella, of which larger one is tinsel or pantonema- tic and the smaller one is whiplash or acronematic type.
10. The reserve foods are commonly laminarin and mannitol. Sucrose and glycerol are also present in some members.
11. They reproduce by all the three means: vegetative, asexual and sexual.
a. Vegetative reproduction takes place by fragmentation. Special reproductive branches, the propagules, are developed in some members of Sphacelariales; those develop to new plants after detachment.
b. Asexual reproduction takes place by zoospores except Tilopteridales, Dictyotales and Fucales. The zoospores produced in unilocular sporangia are haploid, while in pleurilocular sporangia they are diploid.
c. Sexual reproduction ranges from isogamy (Ectocarpales and Sphacelariales) to oogamy (Fucales, Dictyotales and Laminariales) through anisogamy (Cutleriales and Tilopteridales).
12. In most of the members fertilisation is external. Zygote does not undergo meiotic division and on germination it develops diploid thallus.
13. The members show various types of alternation of generations i.e., isomorphic (Ectocarpus), heteromorphic (Laminaria) or diplontic (Sargassum).
The members of Phaeophyceae show two types of life cycle:
1. Diplontic life cycle e.g., Sargassum.
2. Diplohaplontic life cycle.
It is of two types:
i. Isomorphic type e.g., Ectocarpus.
ii. Heteromorphic type e.g., Laminaria.
Classification of Phaeophyceae (Brown Algae):
Fritsch (1935, 45) classified the Class. Phaeophyceae into nine orders. This was also followed by Mishra (1966).
The outline of the classification is:
Class. Phaeophyceae
Order.
1. Ectocarpales e.g., Ectocarpus, Haiothrix.
2. Tilopteridales e.g., Ptilopteris.
3. Cutleriales e.g., Cutlria.
4. Sporochnales e,g. Sporochnus.
5. Desmarestiales e.g., Desmarestia.
6. Laminariales e.g., Laminaria.
7. Sphacelariales e.g., Sphacelaria.
8. Dictyotales e.g., Dictyota.
9. Fucales e.g., Sargassum.
1. Order. Ectocarpales:
Important Characteristics:
i. The plant body is filamentous, branched and heterotrichous, a few are pseudoparenchymatous.
ii. In filamentous form the cells are arranged uniseriately.
iii. The plant body elongates by intercalary growth.
iv. Each cell contains chloroplasts with pyrenoids.
v. Reproduction takes place both asexually and sexually. Asexual reproduction takes place by zoospores formed in pleurilocular sporangia. Sexual reproduction takes place by both isogamy and anisogamy, though anisogamy is rare. Oogamy is absent.
vi. They perform either isomorphic or heteromorphic alternation of generations.
Family. Ectocarpaceae
Important Characteristics:
i. The thalloid plant body is filamentous, branched and heterotrichous.
ii. Sexual reproduction is isogamous.
iii. Alternation of generation is isomorphic.
2. Order. Fucales:
Important Characteristics:
i. The plant body is a sporophytic parenchymatous thallus, grows by single apical cell.
ii. The plant body bears two types of sporangia, the microsporangia and macrosporangia.
iii. Microsporangia behave as antheridia and develop antherozoids, whereas macrosporangia behave as oogonia and develop single egg within each.
iv. Sexual reproduction is oogamous type.
v. The sporangia are borne within special cavities, the conceptacles.
vi. Absence of alternation of generations.
Family. Sargassaceae
Important Characteristics:
i. Plant body is thalloid, much branched and radially or bilaterally symmetrical.
ii. Branching is monopodial.
iii. Growth takes place by three-sided apical cell.
iv. Some branches are modified into leaves, which bear axillary air-bladders.
v. Sex organs are developed in conceptacles, embedded in cylindrical or flattened receptacles.
3. Order. Laminariales:
Important Characteristics:
i. Members are commonly called “kelps”, because of very large size and complexity in structure. Some of the members reach up to 50-60 meters.
ii. They are exclusively marine and commonly grow in sub-littoral and littoral zones of rocky coasts of temperate and polar regions.
iii. Plant bodies are mostly parenchymatous and growth takes place by intercalary, medullary and superficial meristems.
iv. Dominant plant body is very large and sporophytic (2n) in nature. They are usually differentiated into root-like holdfast or hapteron, stalk-like stipe and leafy blades. But the gametophytic plant body is very small and microscopic.
v. Reproduction takes place by both asexual and sexual means.
vi. Asexual reproduction generally takes place by zoospores developed in unilocular sporangia.
vii. Sexual reproduction is of oogamous type. Sex organs i.e. antheridia and oogonia are developed on gametophytic plant.
viii. Plants of this group exhibit heteromorphic alternation of generations.
Family. Laminariaceae:
Important characteristics:
i. The sporophytic plant body is differentiated into holdfast, stipe and blade with high degree of morphological and anatomical differentiation.
ii. Sporangia are developed in distinct groups on both surfaces of the blade.
iii. The gametophytic plants are much reduced and dioecious.
iv. Plants of this group possess heteromorphic, diplohaplontic alternation of generations.