Model Answer
0 min readIntroduction
Cycas is a quintessential representative of the gymnosperms, exhibiting primitive characteristics and providing valuable insights into the evolution of seed plants. The ovule of Cycas is a complex structure crucial for the development of the seed, showcasing adaptations for protection and nourishment of the embryo. Understanding its anatomy is fundamental to comprehending the reproductive strategies of this ancient plant group. The L.S. of the Cycas ovule reveals a unique arrangement of layers and tissues, differing significantly from that of angiosperms.
Longitudinal Section of Cycas Ovule – A Detailed View
The L.S. of a Cycas ovule reveals several distinct layers and structures. The following diagram illustrates these features, followed by a detailed description:
Key Structures and their Functions:
1. Integument(s)
Cycas ovules typically possess two or three integuments. The outer integument is thicker and provides initial protection. The inner integument is thinner and contributes to the formation of the seed coat. A micropyle, a small opening, is present at the micropylar end, facilitating pollen tube entry.
2. Nucellus
The nucellus is the central tissue within the ovule, surrounding the egg cell. It provides nourishment to the developing embryo sac. In Cycas, the nucellus is massive and becomes the primary source of food for the developing embryo.
3. Archegonium
The archegonium is the female reproductive structure within the ovule. It consists of a short neck and a venter containing the egg cell. The neck canal facilitates the passage of the male gametes (sperm) to the egg.
4. Egg Apparatus
The egg apparatus comprises the egg cell and two archegonial cells. The egg cell is the female gamete, which fuses with the male gamete during fertilization.
5. Pollen Chamber
A pollen chamber is formed within the nucellus, resulting from the disintegration of nucellar tissue after pollen tube entry. This chamber provides space for the sperm to reach the archegonium.
6. Resin Canal
Resin canals are present in the nucellus and integuments, containing resinous substances. These canals are thought to provide protection against pathogens and herbivores.
7. Tapetum
The tapetum is the innermost layer of the nucellus, surrounding the embryo sac. It provides nourishment to the developing embryo sac and plays a role in pollen tube guidance.
Table summarizing the layers of Cycas Ovule:
| Layer | Characteristics | Function |
|---|---|---|
| Outer Integument | Thick, protective | Provides initial protection to the ovule |
| Inner Integument | Thinner | Contributes to seed coat formation |
| Nucellus | Massive, nutritive | Provides nourishment to the developing embryo sac |
| Archegonium | Contains egg cell | Site of fertilization |
The process of fertilization in Cycas is unique, involving motile sperm that swim through a fluid-filled pollen chamber to reach the egg. This feature is considered a primitive characteristic among gymnosperms.
Conclusion
The L.S. of the Cycas ovule demonstrates a complex structure adapted for successful reproduction in this ancient gymnosperm. The multiple integuments, massive nucellus, and specialized archegonium all contribute to the protection and nourishment of the developing embryo. Understanding the anatomy of the Cycas ovule provides valuable insights into the evolutionary history of seed plants and their reproductive strategies. Further research into the molecular mechanisms governing ovule development in Cycas could reveal novel insights into plant reproduction.
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.