Chlorophyceae

 

* The members of chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of pigments chlorophyll a and b. The pigments are localised in definite chloroplasts. The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.

* Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.

* Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia,

* The sexual reproduction shows considerable variation in the type and formation of sex cells and -it may be isogamous, anisogamous or oogamous.

* Some commonly found green algae are:

Chlamydomonas, Volvox, Ulothrix, Spitogyra and Chara.

Spirogyra:

* It is an unbranched filamentous green alga of stagnant fresh waters which forms floating masses (supported by bubbles of oxygen) called pond scum. A sheath of mucilage occurs on the outside. It gives a silky touch. Hence Spirogyra is also called water silk or mermaid's tresses.

The thallus is an unbranched and uniseriate filament where cells are arranged in a Single row. In some species hold fast is present (e.g., S. fluviatilis). The cells are elongated and cylindrical. The cell wall is two layered the outer is of pectic substance and the inner of cellulose. The outer part (pectin) dissolves in water to form a mucilaginous sheath. Due to this reason Spirogyra filament's are slippery. Transverse or septum can be plane, colligate (with If-shaped piece), replicate (ring like ingrowths) and unduliseptate (undulate). The protoplast is differentiated into plasma membrane, thin layer of cytoplasm, single nucleus, one (e.g. S.sahni and S. venkataramanni) or many (16 in S.rectispora) ribbon (spiral) shaped chloroplasts (wavy margin) with pyrenoids and a large central vacuole. Nucleus Occurs inside the central vacuole where it Is suspended by means of cytoplasmic strands.

Spirogyra (a) Two filaments (b) and (c) Detailed structure of a cell

* Normally asexual reproduction is absent in Spirogyra. U occurs only occasionally by the formation of akinetes, aplanospores and azygospores (Parthenospores).

*      The sexual reproduction in Spirogyra is called conjugation, It involves the fusion of two morphologically identical, but physiologically dissimilar gametes.

  (a)-(d) Sexual reproduction showing scalar form conjugation in spirogyra

* Life cycle in Spirogyra is haplontic as dominant phase in life cycle is haploid (n) and diploid phase is represented by only zygospore and it undergose R.D. or meiosis (zygotic meiosis).

Ulothrix:

* It is a green filamentous algae found in slow running streams. The common species U. zonata occurs in cold water whereas U. flacca is marine. U. implexa occurs in esturies (where river meats the sea) as lithophytes.

* The Uloihrix reproduces vegetatively, asexually as well as sexually.

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