Algae are plant-like microorganisms present in ditches, ponds, etc. The minute free-floating algae are known as phytoplanktons. Algae comprise over 30,000 species distributed in varied habitat. The study of algae is known as Algology or Phycology. Algae are aquatic and autotrophs. They bear thallus body and contain chloroplast & various photosynthetic pigments. Algae are found in moist places. They show large variation in terms of size. Some are microscopic, hence called microalgae while some can be seen through naked eyes and grow up to a height of 60m, hence called macroalgae. Algae can be unicellular, multicellular or filamentous. Mode of reproduction in algae, can be vegetative, sexual and asexual. Vegetative reproduction occurs through fragmentation. Asexual reproduction take place through spore formation, and sexual reproduction takes place through the formation of flagellated male and female gamete. On the basis of colour, algae can be divided into green algae, brown algae and red algae. Algae that belong to group chlorophyta are green in colour due to the presence of chlorophyll. Their reserve food is starch. Depending upon nature of body, they may be unicellular, Multicellular or Colonial. Chlorella, Chlamydomonas and Desmids are all unicellular green alage. Spirogyra is an example of multicellular green algae. In spirogyra, Asexual reproduction occurs by transverse cell division and Sexual reproduction occurs by process of conjugation. Volvox is a colonial green alga Algae belonging to group cyanophyta are blue- green in colour due to the presence of pigment phycocyanin. Their reserve food is Cyanophycean Starch. Blue green algae are also called cyanobacteria. These are microscopic and autotrophic organisms. Cyanobacteria are found in marine sediments, freshwater lakes and soil. Algae belonging to group Phaeophyta are brown in colour due to the presence of pigment fucoxanthin. Their reserve food is laminarian Starch. Brown alage are mostly found in sea water. Examples of brown algae include Laminaria or kelp, Fucus or rockweed, Sargassum, etc. Algae belonging to group Rhodophyta are red in colour due to the presence of pigment phycoerythrin. Their reserve food is Floridean Starch. They provide agar, which is used for in-vitro cultures. Examples of red algae are Polysiphonia, Chondrus and Grenella. Diatoms, sometimes refereed golden brown algae have silica in their cell wall in the form of two overlapping halves. They are also called “jewels of plant world”. They have silica shell called frustules. Examples of diatoms are Licmophora, Navicula, Chaetoceros, etc.
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