AZOLLA

CLASSIFICATION OF AZOLLA

Kingdom :-  Plantae

Division :- Pteridophyta

Sub-division :- Pteropsida

Class :- Leptosporangiatae

Order :- Salviniales

Family :- Salviniaceae

Genus :- Azolla

Azolla forms red coloured bloom in ditches and ponds. It is found floating freely on the surface of water. Common Indian species is A. pinnata.
Another species A. filiculoides is also known to occur frequently while the third species A. imbricata is found mostly in Eastern Himalayas.

EXTERNAL MORPHOLOGY OF THE SPOROPHYTE OF AZOLLA​

COMMENTS :-

• The plant is a sporophyte. It grows free floating in ditches and ponds
• The plant body is differentiated into stem, roots and leaves.
• The stem is pinnately branched. It is horizontally floating. The branches are extraaxillary. 
• Roots are produced from the lower side of the stem. These remain submerged in water. 
• Leaves cover stem and its branches. These are present in two alternate and overlapping rows.
• Each leaf is divided into two lobes of approximately equal size.
• The upper or aerial lobe is thick and green. It is somewhat obliquely placed and only one of the edges touches the water.
• The thin lower or submerged lobe is nearly colourless. The absorption of water is believed to take place through this lobe.

ANATOMY OF ROOT OF AZOLLA

COMMENTS :-

• The outline of the section is almost circular.
• It shows epidermis, cortex and the stele. 
• Epidermis is the outermost single layer of cells. 
• Cortex consists of 2-8 layers of parenchymatous cells. 
• Endodermis lies inner to cortex. It is made of a single layer consisting of 6 cells. 
• Pericycle that follows is also made of a single layer consisting of 6 cells. 
• Xylem lies in the center. It is represented by two centrally placed metaxylem tracheids. These are surrounded by four small outer groups of protoxylem elements. 
• Phloem consists of only a few elements. These are placed on either sides of the metaxylem elements.

ANATOMY OF STEM OF AZOLLA​

COMMENTS :-

• Outline. Transverse section of the stem is almost circular in outline. 
• It shows epidermis, cortex and stele. 
• Epidermis is the outermost single layer of cells. 
• Cortex is five to eight cells in thickness. The cells are thin walled and parenchymatous without intercellular spaces.
• Stele is centrally located. It is surrounded by single layer of endodermis followed by a single layer of parenchymatous pericyc1e. 
• The central cylinder is protostelic. The vasculature of the stem is greatly reduced in response to aquatic habitat.
• Vascular tissues are represented by about six xylem elements and twice as many phloem elements, in a stele.

ANATOMY OF LEAF OF AZOLLA​

COMMENTS :-

• The upper lobe of leaf is bound on both the sides by upper and lower epidermal cells. 
• Both the layers possess stomata.
• The upper epidermis has many unicellular or bicelled hairs.
• Major portion of the leaf between both epidermal layers is made of palisade-like photosynthetic cells. Large intercellular spaces are present between them. 
• The upper lobe has a large cavity at its base. It opens to the outside through a circular pore. 
• The cavity is filled with the filaments of blue green alga-Anabaena azollae. The alga has a symbiotic relationship with the fern. It fixes atmospheric nitrogen. 
• The pore is later closed by out growths of the tissue of the .margin. It becomes filled with mucilage.

STRUCTURE OF SPOROCARP​

COMMENTS :-

• Sporocarps are borne only on the lowermost leaf of a lateral branch at the end of annual season. 
• Submerged lobe of the leaf bears 2-4 sporocarps. 
• The upper lobe of the fertile leaf forms a hoodlike covering around the sporocarp. 
• The sporocarps are dimorphic i.e. these are of two types: microsporocarps and megasporocarps.
• Larger sized is a microsporocarp and the smaller sized is a megasporocarps.
• Each sporocarp is a sorus covered by indusium. 
• Microsporocarp shows a central raised cushion on which sporangia develop basipetally. Each micro sporangium has one layered jacket. It is followed by tapetum. The cavity encloses 64 microspores. 
• Microsporangium has a multinucleate periplasmodium formed as a result of breakdown of tapetum. Periplasmodium forms four or more quadrately arranged massulae in which spores remain embedded at periphery. 
• The surface of massulae has many anchorshaped barbed hairs called glochidia which help the attachment of massulae to the microspore. 
• Megasporocarp shows a single large megasporangium. It is surrounded by a flaskshaped indusium. It envelops the sporangium completely except for a narrow slit at the apex. 
• Megasporangium is covered by a single layered wall. It encloses a single megaspore.
• Megaspore is surrounded by a hardened vacuolate layer-the perispore. The megaspore wall is hard and ornamented. It is called epispore.
• At the distal end of the megaspore, four quadrately arranged massulae are present. These are formed by the remaining aborted spores and the tapetal cells.

IDENTIFICATION OF AZOLLA

• DIVISION – Pteridophyta
  1. True roots generally present (except in Psilopsida),
  2. True vascular strand present.
• Sub-division :- Pteropsida
  1. Vascular cylinder siphonostelic, with leaf gaps.
  2. Plants macrophyllous, leaves compound, with rachis.
  3. Leaves bear sporangia in sori.
  4. Gametophytes small, green and free living.
• Class :- Leptosporangiatae
  1. Sporangium with a jacket layer one cell in thickness.
  2. Definite number of spores.
• Order – Salviniales
  1. Sporocarp is a single sorus enclosing either megasporangia or microsporangia.
  2. Sporocarp walls formed by the indusia.
• Family – Salviniaceae  (Single family)
• Genus – Azolla
  1. Presence of endophytic blue green algae Anabaena in the leaf
  2. Each leaf divided into two lobes
  3. Megasporocarp with only one megasporangium.

REFERENCES

• https://en.wikipedia.org/wiki/Azolla
• https://depositphotos.com/stock-photos/azolla.html

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