Model Answer
0 min readIntroduction
Gymnosperms, meaning ‘naked seeds’, represent a crucial evolutionary step in plant life, bridging the gap between seedless vascular plants like ferns and the more advanced angiosperms (flowering plants). Cycadales and Ginkgoales are two ancient orders within the gymnosperms, offering valuable insights into plant evolution. Cycadales, resembling palms or ferns, exhibit several primitive traits, while Ginkgo biloba, the sole surviving species of its order, is renowned as a ‘living fossil’ due to its remarkable preservation through geological time. This answer will discuss the primitive features of Cycadales and explain the reasons behind Ginkgo’s designation as a living fossil.
Primitive Features of Cycadales
Cycadales exhibit a unique combination of primitive and advanced characteristics, placing them in a pivotal position in plant phylogeny. Their primitive features are largely reminiscent of their ancestors, the seed ferns (Pteridosperms).
- Morphology: Cycads possess a stout, unbranched trunk (though some species exhibit branching) and a crown of large, pinnately compound leaves resembling those of palms or ferns. This contrasts with the highly branched structure of most other gymnosperms.
- Vascular System: The xylem of cycads is characterized by the presence of tracheids, similar to ferns and other primitive vascular plants, but lacks vessel elements which are a defining feature of most angiosperms and some advanced gymnosperms.
- Reproductive Structures: Cycads are dioecious, meaning they have separate male and female plants. Their reproductive structures are cone-like, but unlike the compact cones of conifers, cycad cones are relatively large and loosely organized.
- Motile Sperm: A remarkably primitive feature of cycads is the presence of motile sperm, which swim to the archegonia (female reproductive structures) for fertilization. This characteristic is shared with ferns and bryophytes but is absent in most other gymnosperms where pollen tubes deliver sperm directly.
- Coralloid Roots: Cycads possess specialized roots called coralloid roots, which harbor symbiotic cyanobacteria (Nostoc). These cyanobacteria fix atmospheric nitrogen, providing the plant with a valuable nutrient. This symbiotic relationship is considered a primitive trait.
- Leaf Venation: The leaf venation in cycads is dichotomous or parallel, resembling that of ferns, rather than the reticulate venation found in most angiosperms.
Why Ginkgo is Considered a Living Fossil
Ginkgo biloba is often referred to as a ‘living fossil’ due to its remarkable morphological stasis – it has remained virtually unchanged for over 270 million years. This designation stems from several key factors:
- Ancient Lineage: Ginkgoales were a dominant group of plants during the Permian and Mesozoic eras. However, most species went extinct, leaving Ginkgo biloba as the sole surviving member of its order.
- Morphological Conservatism: Ginkgo biloba retains many primitive characteristics of its ancestors, including fan-shaped leaves with dichotomous venation, similar to those found in fossil Ginkgoales.
- Lack of Close Relatives: The absence of closely related species makes it difficult to assess the extent of evolutionary change within the Ginkgoales lineage. Its unique position in the plant kingdom highlights its ancient origins.
- Slow Evolutionary Rate: Compared to many other plant groups, Ginkgo has exhibited a remarkably slow rate of morphological evolution over millions of years.
- Resistance to Extinction: Ginkgo survived major extinction events, including the Permian-Triassic extinction (the ‘Great Dying’) and the Cretaceous-Paleogene extinction (which wiped out the dinosaurs). This resilience suggests inherent adaptive advantages.
Comparison between Cycadales and Ginkgoales
| Feature | Cycadales | Ginkgoales (Ginkgo biloba) |
|---|---|---|
| Leaf Shape | Pinnately compound, resembling palms/ferns | Fan-shaped, dichotomously veined |
| Sperm Motility | Motile sperm | Motile sperm |
| Vessel Elements | Absent | Absent |
| Coralloid Roots | Present (symbiotic with Nostoc) | Absent |
| Current Diversity | Approximately 140 species in 11 genera | One species (Ginkgo biloba) |
Conclusion
Both Cycadales and Ginkgoales represent important relics of ancient plant lineages. Cycadales showcase a blend of primitive and advanced traits, providing insights into the evolution of gymnosperms. Ginkgo biloba, as a ‘living fossil’, stands as a testament to the power of morphological stasis and resilience in the face of environmental change. Studying these groups is crucial for understanding the evolutionary history of plants and the processes that drive adaptation and extinction. Their continued existence offers valuable opportunities for research in areas such as plant physiology, genetics, and conservation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.