Answer: Ophioglossum

Gradate sori, where sporangia mature sequentially from the apex (tip) to the base, are characteristic of Ophioglossum among the given options.​

Option Analysis

Marsilea

Marsilea, a heterosporous water fern, produces sporangia within specialized sporocarps rather than sori. It lacks true gradate sori, with megasporangia and microsporangia developing separately inside bean-shaped structures.​

Selaginella

Selaginella, a lycopod (clubmoss), forms strobili with sporophylls bearing microsporangia and megasporangia. It does not produce sori at all, as sori are fern-specific structures on fronds.​

Pteris

Pteris exhibits marginal sori arranged linearly along leaflet edges, protected by false indusia. These are typically mixed sori, with sporangia of mixed ages maturing irregularly, not in a strict gradate sequence.​

Ophioglossum

Ophioglossum (adder’s-tongue fern) features gradate sori on separate fertile spikes. Sporangia develop in a linear or horseshoe-shaped cluster, ripening progressively from the top downward (apex to base), a primitive gradate type.

Gradate sori can be seen in Ophioglossum, distinguishing it from Marsilea, Selaginella, and Pteris through sequential sporangial maturation from apex to base on fertile spikes. This primitive sorus type aids spore dispersal in these eusporangiate ferns.​

Sorus Types in Pteridophytes

Sori evolve from simple (simultaneous maturation) to gradate (sequential apex-to-base) to mixed (irregular). Gradate sori represent an intermediate evolutionary stage.

• Marsilea: Sporocarps (no sori); heterosporous.​

• Selaginella: Strobili (no sori); lycophyte.​

• Pteris: Marginal, mixed sori with false indusium.​

• Ophioglossum: Linear gradate sori on spikes, eusporangiate.​