Understanding Sphenopsida and its Living Members

The question asks to identify the only living representative of the plant group Sphenopsida that exhibits specific characteristics related to its rhizome and branching pattern. To answer this, we need to know which plants belong to Sphenopsida and examine their structures.

Identifying the Sole Living Sphenopsida Member

Sphenopsida is a class of Pteridophytes (vascular plants that reproduce via spores and do not produce seeds or flowers). While this group had many diverse forms in prehistoric times, most are now extinct. The only genus that has survived to the present day is Equisetum. Due to its long evolutionary history and having only one living genus, Equisetum is often referred to as a living fossil.

Features of the Equisetum Rhizome

The question specifically describes a plant with an underground, creeping, and perennial rhizome that gives off both aerial and underground branches. Let's examine the structure of Equisetum:

• Rhizome: Equisetum possesses a well-developed, underground, horizontal stem called a rhizome.
• Nature of Rhizome: This rhizome is typically creeping (grows horizontally just below the soil surface) and perennial (lives for many years).
• Branching: The rhizome bears buds that develop into aerial shoots (stems visible above ground) and also gives rise to underground branches and roots. The aerial shoots can be fertile (spore-bearing) or sterile (photosynthetic).

These characteristics perfectly match the description given in the question regarding the underground, creeping, and perennial rhizome producing aerial and underground branches.

Analyzing Other Options

Let's briefly look at the other options provided to understand why they are not the correct answer:

• Adiantum: This is a genus of ferns, commonly known as maidenhair ferns. Ferns belong to the class Pteropsida (or Polypodiopsida), not Sphenopsida. They have rhizomes, but they are distinctly different from Equisetum and are not the sole living representative of Sphenopsida.
• Dryopteris: This is another genus of ferns, known as wood ferns. Like Adiantum, Dryopteris belongs to the Pteropsida and is not a member of Sphenopsida.
• Selaginella: This genus is commonly known as spike moss or little club moss. Selaginella belongs to the class Lycopsida (or Lycopodiopsida). It is also a Pteridophyte but is evolutionarily distinct from Sphenopsida.

Therefore, based on the classification and the morphological features described, Equisetum is the only option that is a living representative of Sphenopsida and fits the description of the rhizome structure.

Conclusion on Sphenopsida Representation

The only living genus of Sphenopsida is Equisetum. Its characteristic underground, creeping, and perennial rhizome that produces both aerial and underground branches aligns precisely with the features mentioned in the question. The other options belong to different groups of Pteridophytes.

Revision Table: Key Sphenopsida Facts

Additional Information: Pteridophyte Groups

Pteridophytes, or vascular cryptogams, are a diverse group of plants that includes ferns and their allies. They are characterized by having vascular tissues (xylem and phloem) but reproducing by spores rather than seeds. They are broadly classified into several groups, including:

• Lycopsida (Lycophytes): Includes clubmosses, spikemosses (like Selaginella), and quillworts. They typically have microphylls (small leaves with a single vein).
• Sphenopsida (Equisetophytes): Represented today only by the genus Equisetum (horsetails). Characterized by jointed stems, whorled leaves (reduced and scale-like), and sporangia borne in strobili at the stem tips.
• Pteropsida (Ferns): The largest group, includes ferns, horsetails (sometimes placed here), and whisk ferns. Ferns are typically recognized by their large, often compound leaves (megaphylls) called fronds and sporangia usually clustered in sori on the underside of the fronds. Adiantum and Dryopteris are examples of ferns.

Understanding these different groups helps clarify why Equisetum is unique as the sole survivor of the Sphenopsida lineage with the described rhizome features.