Ephedra and Gnetum are exceptions to gymnosperms as they show

Options

A. Double fertilization
B. Siphonogamy
C. Aggregation of sporophylls
D. Presence of ovules .

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks why Ephedra and Gnetum are considered exceptions among gymnosperms. We need to identify the unique characteristic they exhibit that differentiates them from typical gymnosperms.

2. **Identifying Key Characteristics of Gymnosperms**: Gymnosperms are primarily characterized by their method of reproduction, which typically involves the process of siphonogamy (the transfer of pollen to the ovule) and does not include double fertilization.

3. **Researching Ephedra and Gnetum**: Both Ephedra and Gnetum are genera within the gymnosperms. Notably, they have been studied for their reproductive processes, which reveal some unique features.

4. **Understanding Double Fertilization**: Double fertilization is a process where one male gamete fertilizes the egg, while the other male gamete fuses with a second nucleus (the polar nucleus) to form the triploid endosperm. This is a characteristic feature of angiosperms (flowering plants).

5. **Citing Scientific Evidence**: According to research by scientists such as Charmichael and Friedman, it has been observed that in Gnetum, two male gametes from the pollen tube fuse with different nuclei. Similarly, in Ephedra, one male gamete fuses with the egg, and the other fuses with the venter canal nucleus.

6. **Conclusion**: Based on the evidence, we conclude that Ephedra and Gnetum exhibit double fertilization, which is atypical for gymnosperms. Therefore, they are exceptions to the general characteristics of gymnosperms.

7. **Final Answer**: The correct answer is **A) Double fertilization**.

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