(c) Give an account of structural variation in the megasporophylls of different Cycas species with illustrations. Add a brief note on the primitive features in Cycas.

Structural Variation in Megasporophylls of Different *Cycas* Species

Megasporophylls in *Cycas* are modified leaves that bear ovules and are distinct from the compact female cones seen in most other gymnosperms. They typically consist of a proximal stalk, a central fertile region bearing exposed ovules, and a distal, sterile, leafy lamina. The primary variations among different *Cycas* species lie in the morphology of this distal lamina and the number of ovules borne on the fertile part. These variations reflect an evolutionary trend from more fern-like, dissected structures to more reduced, entire forms.

The megasporophylls are generally covered in yellow or brown woolly hairs, especially when young, and can reach considerable lengths. The ovules are typically arranged in two rows on the stalk-like portion of the megasporophyll, and their micropyles usually point away from the rachis and the stem.

Key Species and Their Megasporophyll Variations:

1. *Cycas revoluta* (Sago Palm):
   • Lamina: The upper, sterile part of the megasporophyll is highly dissected and pinnate, giving it a comb-like or feathery appearance. The tip of each pinna is generally acute.
   • Ovules: Typically bears a relatively high number of ovules, ranging from 2 to 12.
   • Illustration Description: Imagine a large, modified leaf with a long stalk. The upper portion is deeply cut into numerous narrow, pointed lobes, resembling a comb. Along the margins of the central, broader part, multiple ovules are visible.
2. *Cycas circinalis* (Fern Palm):
   • Lamina: In contrast to *C. revoluta*, the distal lamina of *C. circinalis* is less dissected, often only slightly lobed, or nearly entire with dentate margins. It appears broader and less incised.
   • Ovules: Usually bears 4 to 8 ovules.
   • Illustration Description: Picture a megasporophyll with a less frilly or more solid upper part. The lamina might have wavy or coarsely toothed edges rather than deep, comb-like divisions. The ovules are present in a similar arrangement as *C. revoluta*.
3. *Cycas rumphii*:
   • Lamina: Exhibits intermediate features between *C. revoluta* and *C. circinalis*. The lamina is moderately dissected, not as deeply pinnate as *C. revoluta*, nor as entire as *C. circinalis*. The upper sterile part is longer than it is broad, with dentate margins.
   • Ovules: Typically bears 4 to 6 ovules.
   • Illustration Description: Envision a megasporophyll where the upper leafy portion shows some lobing or serration, but these cuts are not as deep or numerous as in *C. revoluta*. It provides a transitional form in terms of dissection.
4. *Cycas beddomei*:
   • Lamina: The megasporophylls are ovate-lanceolate with dentate margins.
   • Ovules: Specific number not explicitly mentioned but would align with the general range for Cycas (2-12).
   • Illustration Description: A megasporophyll with a broader, more ovate shape in the sterile region, having distinct teeth along its edges.

The variations in the sterile apical lamina are significant as they indicate an evolutionary reduction from a more leaf-like structure, reminiscent of ancestral seed ferns, towards the more compactly organized strobili found in other cycads and gymnosperms.

Primitive Features in *Cycas*

*Cycas* is widely regarded as a "living fossil" due to the presence of several primitive characteristics that connect it to ancient plant forms, particularly seed ferns and pteridophytes. These features offer crucial insights into the evolutionary history of seed plants.

• Leaf-like Megasporophylls:
  • Unlike most other gymnosperms where ovules are borne within compact female cones (strobili), *Cycas* megasporophylls are large, leaf-like structures arranged spirally and loosely at the stem apex. They do not form a true cone, reflecting a less specialized and more primitive condition akin to the fertile fronds of some ferns. This is considered a direct evolutionary link to seed ferns (Pteridospermales).
• Motile Sperms (Zooidogamy):
  • *Cycas* possesses large, multi-flagellated sperm cells that are motile and swim through a liquid medium (pollen tube fluid) to reach and fertilize the egg. This mode of fertilization, known as zooidogamy, is a primitive trait shared with lower plants like bryophytes and pteridophytes, and very few other gymnosperms (e.g., *Ginkgo*). Most advanced gymnosperms and all angiosperms exhibit siphonogamy, where non-motile sperm are delivered directly to the egg via a pollen tube.
• Presence of Archegonia:
  • The ovule of *Cycas* contains well-developed archegonia, which are primitive female reproductive organs. Archegonia are characteristic of bryophytes, pteridophytes, and some gymnosperms, but are absent in angiosperms. Their presence in *Cycas* signifies a basal position in the evolutionary lineage of seed plants.
• Large Ovules:
  • The ovules of *Cycas* are among the largest in the plant kingdom, a feature that can be considered primitive, reflecting a less efficient, high-resource investment strategy in reproduction.
• Slow Growth and Long Lifespan:
  • *Cycas* species are known for their extremely slow growth rate and exceptionally long lifespans, with some individuals living for hundreds or even thousands of years. This slow metabolic rate and extended longevity are often associated with ancient plant lineages.
• Vascularization:
  • The stem anatomy of *Cycas* exhibits primitive characteristics, such as a large pith and cortex with scanty woody tissue. The xylem of cycad seedlings often comprises scalariform tracheids, a primitive type of conducting cell.
• Coralloid Roots with Cyanobacterial Symbiosis:
  • *Cycas* develops specialized apogeotropic (growing against gravity) coralloid roots that harbor symbiotic cyanobacteria (like *Nostoc* and *Anabaena*). These cyanobacteria fix atmospheric nitrogen. While symbiosis is not inherently primitive, the specific morphology and structure of these roots are considered an ancient adaptation within the cycads.

These primitive features collectively underscore *Cycas*'s evolutionary significance, placing it as a critical link in the phylogenetic tree of plants, bridging the gap between spore-reproducing plants and advanced seed plants.