Give an account of male gametophyte development in Gnetum. State the angiosperm characters shared by Gnetum.

Male Gametophyte Development in Gnetum

The male gametophyte development in Gnetum is a complex process that deviates significantly from the typical gymnosperm pattern. It can be divided into the following stages:

1. Microsporogenesis

Microsporogenesis begins with the development of microsporangia within the male cones (microstrobilus). Each microsporangium contains numerous microsporocytes (pollen mother cells). These microsporocytes undergo meiosis, resulting in the formation of tetrads of haploid microspores.

2. Microgametogenesis

Each microspore undergoes mitotic division to form an immature pollen grain. This is where Gnetum differs significantly from typical gymnosperms. Instead of forming a simple pollen grain with a prothallial cell and generative cell, the microspore divides asymmetrically. This results in a smaller vegetative cell and a larger generative cell. The vegetative cell is relatively inconspicuous.

3. Formation of the Pollen Tube Cell and Generative Cell

The generative cell divides mitotically to produce two male gametes (sperm cells). Crucially, the vegetative cell develops into a pollen tube, a feature typically associated with angiosperms. This pollen tube grows through the micropyle of the ovule to deliver the sperm cells to the egg cell. This is a significant departure from the motile sperm cells characteristic of most other gymnosperms.

4. Pollen Grain Structure

The mature pollen grain of Gnetum is relatively large and possesses a complex structure. It consists of the vegetative cell, the two sperm cells, and a thick exine (outer wall) providing protection. The presence of a well-developed pollen tube cell is a key characteristic.

Angiosperm Characters Shared by Gnetum

Gnetum exhibits several characteristics that are typically associated with angiosperms, making it a unique gymnosperm. These include:

• Presence of Vessels: Gnetum possesses vessels in its xylem, a feature generally absent in other gymnosperms but common in angiosperms. Vessels are more efficient in water transport than tracheids.
• Pollen Tube: As described above, the development of a pollen tube to deliver sperm cells to the egg is a characteristic shared with angiosperms. This eliminates the need for motile sperm.
• Compound Pistils: In some Gnetum species, the ovules are enclosed within a structure resembling a compound pistil, although it is not homologous to the angiosperm carpel.
• Opposite Phyllotaxy: The arrangement of leaves (phyllotaxy) in Gnetum is often opposite, a pattern more common in angiosperms than in most gymnosperms.
• Presence of Pericyclic Fibers: The vascular bundles in Gnetum contain pericyclic fibers, a feature also found in many angiosperms.
• Reticulate Venation: Some Gnetum species exhibit reticulate (net-like) venation in their leaves, a characteristic typically associated with dicotyledonous angiosperms.

These shared characteristics have led to hypotheses suggesting a close evolutionary relationship between Gnetum and angiosperms, although the exact phylogenetic position of Gnetum remains a subject of ongoing research.