Model Answer
0 min readIntroduction
The term ‘living fossil’ refers to extant species that closely resemble ancient ancestors known from the fossil record, exhibiting minimal morphological change over long geological timescales. *Ginkgo biloba* is arguably the most iconic example of this phenomenon. Originating in the Permian period (over 299 million years ago), it represents the sole surviving species of the Ginkgophyta division, a group that was once widespread. Its remarkable persistence, despite dramatic climatic and geological shifts, is intrinsically linked to its unique reproductive biology, which retains characteristics of its early ancestors, setting it apart from most modern seed plants.
Ancient Lineage and Morphological Stasis
Fossil evidence demonstrates that *Ginkgo* ancestors were abundant during the Mesozoic era, the ‘age of reptiles’. However, with changing climates and the rise of angiosperms, the genus experienced a significant decline. *Ginkgo biloba* survived only in isolated regions of China, where it was cultivated in Buddhist temples. This preservation through human intervention, coupled with its inherent resilience, explains its continued existence today. Morphologically, modern *Ginkgo* trees are remarkably similar to fossils dating back 150 million years, showcasing exceptional evolutionary stasis.
Reproductive Structures: A Glimpse into the Past
The reproductive biology of *Ginkgo* is particularly noteworthy for its primitive characteristics. Unlike most seed plants that rely on wind pollination, *Ginkgo* exhibits a unique mode of fertilization involving motile sperm.
Motile Sperm
*Ginkgo* is unique among extant seed plants in possessing flagellated sperm cells. These sperm, relatively large and multi-flagellated, swim towards the archegonia (female reproductive structures) within the ovules. This feature is reminiscent of the reproductive strategies employed by ferns and cycads, highlighting *Ginkgo’s* evolutionary link to these earlier plant groups. The sperm are released during a specific time window, and successful fertilization depends on their ability to navigate to the ovules.
Ovule and Seed Development
The ovules of *Ginkgo* are not enclosed within an ovary, a characteristic feature of angiosperms. Instead, they are borne on stalks and are exposed during pollination. Following fertilization, the ovule develops into a large, fleshy seed with a distinctive outer layer. These seeds are dispersed by wind and possess a strong, unpleasant odor, attracting animals that aid in dispersal. The seed structure itself is relatively simple, lacking the complex seed coat layers found in many modern plants.
Pollination Mechanism
*Ginkgo* relies on wind pollination, but the process is somewhat unusual. Male trees produce pollen cones that release pollen grains containing the motile sperm cells. The pollen grains are carried by the wind to the female trees, where they land near the micropyle of the ovule. The pollen grain then germinates, releasing the motile sperm which swim to fertilize the egg.
Survival Advantage of Primitive Traits
While seemingly disadvantageous in a modern context, the primitive reproductive traits of *Ginkgo* may have contributed to its survival. The motile sperm, while requiring a moist environment for fertilization, may have provided a selective advantage in the past when conditions were more humid. Furthermore, the robust seed structure and the production of secondary metabolites (like ginkgolides) offer protection against herbivores and pathogens. The limited genetic diversity within the species, while a concern for long-term adaptability, hasn’t yet proven detrimental to its survival.
Conclusion
*Ginkgo biloba’s* designation as a ‘living fossil’ is well-deserved, stemming from its ancient lineage and remarkable morphological conservatism. Its unique reproductive structures, particularly the motile sperm and exposed ovules, represent a fascinating glimpse into the evolutionary history of seed plants. While facing challenges in a changing world, *Ginkgo’s* resilience and unique adaptations continue to ensure its survival, making it a valuable subject for paleobotanical and evolutionary studies.
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.