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Sorus Definition

In Botany, Sorus, (sori in plural), is a yellow-brown cluster of structures that produce spores (sporangia). It is found usually on the lower surface of the leaves of ferns. Sorus is sometimes surrounded by a secondary external layer in some varieties of fungi and lichens. Fern sorus is easily identified by its brownish-yellow colour. 

In house ferns, when sori develop on the leaves and are in a young stage, are mistaken for tiny insects and for a fungus disease when they develop further. In reality, sorus meaning is organs that are important for the normal reproduction process of the plant.

In smut and rust fungi, sorus meaning is a spore that is mass-produced on the infected plant’s leaf. In certain algae species, the definition of sori includes reproductive structures. In red algae, it sometimes takes the form of depression into the thallus. The sorus is made up of small, circular bodies known as sporangia. Within the clusters, the sporangia have several haploid spores. 

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During its development stage, sorus in some plants is covered and protected by a thin flap or a scale of tissue called an indusium. It forms an umbrella-like cover over the sorus. It protects the sporangial cluster from drying, exposure, drying, and other dangers. Sometimes, in the absence of an indusium, sori are covered by protective structures like the edge of a leaf. These structures partially surround the sporangia that are considered ‘naked’ in such cases. When the sporangia are matured, the indusium shrivels to allow for the unhindered release of the spores. These spores are then released when the sporangia bursts. After they are released from the sporangia, these spores produce and grow into the gametophytic generation.

The arrangement, location, and shape of sorus are used by botanists to identify their different variants. In some plants, sori may be circular, linear, or arranged in oblique rows to the costa. The location of sori may be slightly away from the margin on the frond lamina. Fern taxa can be distinguished in some cases, by the presence or absence of indusium.

Some species of ferns produce a unique frond that is fertile and generates only sori. Such fronds are not photosynthetic and can be easily distinguished from sterile fronds due to their appearance. 

Evolution of Sori

There are broadly three stages in the progressive evolution of sori:

  1. Simple Sporangia Clusters: They are more or less separate (Gleicheniaceae) or are coalesced (family Marattiaceae). They all mature at the same time

  2. Graduate Sporangia Clusters: The outermost clusters mature first and the innermost mature at the last 

  3. Mixed Sporangia Clusters: There is a presence of all ages with the younger ones arising from the same meristematic zones just like the older ones. 

This sequence or change has an adaptive significance and is most likely related to the spore production duration. The more advanced sori have a mixed character and it extends the period beyond that for simple simultaneously maturing sori or solitary sporangia.

Adiantum Sorus

Adiantum is a fern species found in the temperate and tropical regions of the world. They are popularly known as ‘Walking fern’ or ‘Maiden hair fern’. Some Indian species of Adiantum include Adiantum pedatum, Adiantum capillus-veneris, Adiantum venustum and Adiantum caudatum. The leaves of Adiantum have marginal sori covered with a faux indusium. Reproduction in Adiantum occurs through the production of spores as it is homosporous. 

The sporangia produce these spores. Although the Adiantum sorus is marginal, a protective membranous structure called false indusium is formed by the reflex margins of the pinna. The development of sporangium in Adiantum fern is of leptosporangiate type. Adiantum sorus does not have any distinct shape and is considered a mixed type sori. 

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FAQs on Sorus

1. What is Sori Plants Definition?

Ans. Clusters of sporangia found on the undersurface of ferns are called sori (singular: sorus). Sori are formed by reproduction and have different sizes and shapes.

2. How Does Sorus Help in the Reproduction Process in Adiantum?

Ans. A mature sporangium in Adiantum has an elliptical or spherical single-layered structure called a capsule and a multicellular stalk. This capsule bears haploid spores and its wall is differentiated into thin-walled stomium and thick-walled annulus. When the sporangium gets mature, it bursts, releasing the spores. These spores germinate and produce a prothallus by undergoing repeated division. The prothallus is green, flat, and heart-shaped. Sex organs called archegonia and antheridia develop on the prothallus which represents the gametophytic phase and is monoecious. Antheridia releases multiflagellate antherozoids. They swim in the water to reach the egg of the archegonium for achieving fertilization.

3. What is Indusium?

Ans. Indusium is the rolled-over leaf margin, a protective cover formed over sorus. The sporangia form and mature under the indusium. This type of indusium is called a false indusium. A true indusium is unique and has a separate formation. Its structural origins are not clear and it forms a paper-like covering over the sorus. A commonly found type of indusium among the Cyatheaceae family is shaped like a cup and it rises around the base of the sorus. This indusium often encloses the sorus until the sporangia are mature. In some variants, the marginal sori are protected by a valvate or two-lipped indusium (e.g., Dicksonia, Dennstaedtia, and Hymenophyllum). If the sori fuse laterally to form coenosori or continuous lines, the indusia also get fused.

4. How Do Sori Help in Fern Classification?

Ans. Sori and Sporangia help in fern classification by providing some important characters such as soral structure. Almost one-third to a half of the fern species of ferns fall under one of the six soral arrangements: 

  1.  A linear arrangement of sporangia along the veins of the leaf. This arrangement avoids the leaf area between the veins (found in genus Pityrogramma). 

  2. A line of sporangia along the edge of the leaf. This line is protected by a modified rolled-over laminar margin (genus Pteris). 

  3. Naked (without an indusium) and round sori (genus Polypodium). 

  4. An arrangement of large-size sori spread over the entire undersurface region of the blade or pinna (genus Acrostichum) 

  5. An oblong or linear sorus along a vein. It is covered from one side by a narrow indusium (genus Asplenium).

  6. Round sorus covered with a kidney-shaped or shield-like indusium (genus Dryopteris).

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