F.E. Fritsch (1935, 1945) in his book “Structure & reproduction of algae” gave a very comprehensive account of algae.
He divided algae into 11 classes (suffix-phyceae), mainly on the basis of pigmentation, thallus-structure, reserve food, flagellation & modes of reproduction.
The 11 classes are:
i. Chlorophyceae (Green algae)
ii. Xanthophyceae (Yellow- green algae)
iii. Chrysophyceae (Golden Brown algae)
ix. Phaeophyceae (Brown algae)
x. Rhodophyceae(Red algae)
xi. Cyanophyceae (Myxophyceae) – BGA
R.H. Whittaker grouped only Chlorophyceae, Phaeophyceae & Rhodophyceae under plant kingdom, Cyanophyceae under kingdom-Monera and the rest under kingdom-Protista.
1. Chlorophyceae (Green algae) (Fig. 5.2):
1. About 7000 species are known, mostly freshwater except a few (- 10%) marine forms.
2. The members are multi-cellular; but may be unicellular, colonial or coenocytic.
3. Chloroplasts contain photosynthetic pigments (Chl- a, b, carotenes and xanthophylls) similar to those of land plants.
4. Cell wall made up cellulose.
5. Reserve food is starch.
6. Sexual reproduction is isogamous, anisogamous, and oogamous type. Examples: Spirogyra, Ulothrix, Caulerpa,VoIvox, Acetabularia, Chlorella etc.
2. Phaeophyceae (Brown Algae) (Fig. 5.3):
1. It includes about 2000 species, mostly marine.
2. Unicellular forms absent.
3. They appear brown due to large amount of brown coloured xanthophyll pigments called fucoxanthin (C40H56O6).
4. Photosynthetic pigments include Chl-a, c, carotenes &xanthophylls.
5. The plant body is a thallus differentiated into holdfast, stipe and lamina (blade or frond). Photosynthetic lamina is annual while stipe is perennial. Holdfast helps in anchorage. A few species are free-floating e.g. Fucus (rockweed), Sargassum (gulf weed). Sargassum covered thousand of hectors in the Sargasso Sea in the North Atlantic Ocean and it is a menace to shipping as they get attached to the bottom of ships.
6. The larger forms of brown algae are called kelps or sea weeds e.g. Macrocystis (30-60m, the largest sea plant), Nereocystis (20-30m.). The giant kelps contain air vesicles or bladder for buoyancy.
7. Cell wall composed of a mixture of polysaccharides like cellulose, pectose and algin (non- sulphated phycocolloids). Chemically, algin is the calcium salts of alginic acid (a major phycocolloid). Phycocolloids are complex polysaccharides that store in the cell wall of algae, protect them from desiccation and prevent drying or freezing (in winter) when exposed to air in low tide.
8. Food reserve is laminarin and mannitol.
3. Rhodophyceae (Red Algae) (Fig. 5.4):
1. About 5000 species are known, mostly marine except a few fresh water forms (Batrachospermum)
2. They appear red due to phycoerythrin (red pigment, C34H46O8N4) & phycocyanin (the blue pigment, C34H46O8N4). These pigments absorb blue-green region of spectrum i.e. 480-520 nm which can penetrate greater depth of water. Hence, the red algae are the deepest growing algae in the seas where other photosynthetic forms cannot grow.
3. Red algae appear more red in deep water because of excess phycoerythrin than chlorophyll is formed. When growing in shallow water, they appear green due to more chlorophyll. This property of change in pigmentation (colour) is called chromatic adaptation (Gaidukov phenomenon).
4. Nutrition is photoautopophic, except some colourless & parasitic forms like Harveyella which live on other red algae.
5. Reserve food is floridean starch.
6. Cell wall composed of cellulose, pectin & sulphated phycocolloids (agar, carageenin & funori).
7. Vegetative reproduction occurs by fragmentation & regeneration of holdfast. Some reproduce asexually by spores. Sexual reproduction is oogamous type.
8. The thallus of red algae may be unicellular (Porphyridium), filamentous (Batrachospermum, Polysiphonia), pseudofilamentous (Astocystis), parenchymatous (Porphyra), lace-like (Gelidium), ribbon-like (Chondrus) etc.
Green Algae as Ancestors of Land Plants:
Because of morphological, cytological and biochemical similarities, and phylogenetic evidences, it is now believed that green algae are the ancestors of land plants.
Some of the points in support of this view are briefly given below:
1. Both green algae and land plants possess the same kind of photosynthetic pigments, i.e. chlorophyll a, chlorophyll b, and carotenoids.
2. Cell wall contains cellulose and pectose in both.
3. In both the groups reserve food material is starch.
4. The structure of the flagella is similar in motile forms of both the groups.
5. Definite tendency is seen among the members of chlorophyceae to migrate towards land and lead life like land plants.
Land plants have advanced over the members of green algae along the line of folio wing adaptations:
1. Increased structural complexity of the plant body.
2. Highly developed reproductive organs with special adaptation to protect the gametes.
3. Protection of zygote, embryo formation, and great development of post-fertilization stages.