Question

Describe the evolution of sorus in pteridophytes.

Answer 1

Hint: Sorus, (sori in plural), is a yellow-brown cluster of sporangia which produce spores. It is usually found 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. In some species, they are protected during development by a scale or film of tissue called indusium, which forms an umbrella-like cover. It may be unprotected and necked.

Complete answer:
Sorus is a group of sporangia produced on the abaxial surface of the sporophylls in ferns. In some plants, sori may be circular, linear, or reniform. The location of sori may be slightly away from the margin on the frond lamina. Fern taxa can be distinguished in some cases on the basis of the presence or absence of indusium. They originate in different ways;
a) Marginal sorus
b) Intra-marginal
c) Superficial / abaxial sorus
During its development stage, sorus in some plants is covered and protected by a thin flap or a scale of tissue called an indusium, which 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, which are considered as ‘naked’ in such cases. When the sporangia are matured, the indusium shrinks to allow for the unhindered release of the spores. These spores are then released when the sporangia bursts. After release, those spores produce and grow into the gametophytic generation.
There are broadly three stages in the progressive evolution of sorus:
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 sorus has a mixed character and it extends the time period beyond that for simple simultaneously maturing sorus.

Additional Information:
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 ‘Maidenhair fern’. The leaves of Adiantum have marginal sori covered with a faux indusium. Reproduction in Adiantum takes place 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 the Adiantum fern is of the leptosporangiate type. Adiantum sorus does not have any distinct shape and is considered a mixed type of sori.

Note:
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. But in reality, sorus indicates the organs which are necessary for the normal reproduction process of the plant. Some species of ferns produce a unique frond, i.e., fertile and generates only sori. Such fronds are not photosynthetic and can be easily distinguished from sterile fronds due to their appearance.