Pinnularia

Pinnularia is a fresh water alga and a type of Bacilliophyta (diatoms).

Habitat

Pinnularia is a fresh Water alga, usually found in ponds and moist soil.They can also be found in springs, estuaries, sediments and in the ocean.

General Structure

Cell Wall and Valves

Pinnularia are unicellular. The cell is elongated and elliptical. The cell wall is chiefly composed of pectic substances and is impregnated with silica. Therefore, their wall becomes very hard. Their wall is composed of two halves called valves. These halves overlap like petri dish. The margins of the two valves are covered by a connecting band called cingulum. The two valves with their inner protoplasts are called frustule. The frustule has two views. The surface view is called valve view and band view is called girdle view. The outer larger valve is called epitheca. The smaller inner valve is called hypotheca. The cell is covered by mucilaginous layer. The surface of valve has minute pores or pits. It produces characteristic markings on the surface of valves. These markings are bilaterally symmetrical. The marking free area is called axial field. The markings are arranged in linear rows. The axial field contains a longitudinal slit raphe. The raphe is not continuous throughout the length of the valve. It has circular thickening called central nodules in the centre. Similar Thickening also present at the outer ends of raphe These are called polar nodules.

Cell structure

Cytoplasm is arranged in parietal layer. It is called primordial Utricle. A large central vacuole is present inside the cell. A single nucleus is suspended in the centre of the vacuole by a transverse cytoplasmic bridge. Two chloroplasts are present along the sides of the cells. They contain diatomin pigment. One or two pyrenoids are present in the chloroplast. But they do not synthesize starch. Their storage compound is fat.Presence of Chrysolaminarin & Volutin(protein)

Reproduction

Vegetative Reproduction

Vegetative reproduction in diatoms occurs by simple cell division. Nuclear division occurs by mitosis and cell divides into two parts. Each half receives one half of the parent cell. It synthesize new valve. New valve fits into the parent valve. Thus new valves are always smaller than the parent halves. Thus one generation gradually become smaller in size. It reaches to minimum size. Then its size is restored by auxospore formation. But second generation remains of same size.

Auxospore formation

The protoplast of cell secretes a large amount of mucilage. therefore, two valves are separated from each other. It separates the inner protoplast. The protoplast grows to its maximum size. It secrets silica rich pectic membrane. This membrane is called perizonium. New valves are secreted inside the perizonium. Nuclear division does not occur. Therefore, auxospores are formed by parthenogenesis.