In scalariform conjugation, the fusing gametes of Spirogyra are morphologically dissimilar.

Explanation:
Scalariform conjugation is a type of sexual reproduction that occurs in filamentous green algae, specifically in the genus Spirogyra. This process involves the fusion of two morphologically distinct gametes, known as the "+" (plus) and "-" (minus) gametes.

Here's a detailed explanation of why the correct answer is option 'A' (morphologically dissimilar):

Morphological Differences:
1. Size: The "+" and "-" gametes of Spirogyra vary in size. The "+" gamete is smaller and possesses a rounded tip, while the "-" gamete is larger and has a conical tip.
2. Shape: The shape of the gametes is also different. The "+" gamete is cylindrical, whereas the "-" gamete is fusiform (spindle-shaped).
3. Projections: The "+" gamete possesses several long, hair-like projections called cilia, which aid in movement and attachment. In contrast, the "-" gamete lacks cilia.

Physiological Similarities:
Despite the morphological differences, both gametes of Spirogyra are physiologically similar. They are both motile and capable of independent movement through the water. Both gametes contain chloroplasts, enabling them to carry out photosynthesis.

Importance of Morphological Dissimilarity:
The morphological dissimilarity between the gametes of Spirogyra is essential for successful sexual reproduction. It ensures that only compatible gametes fuse during conjugation.

During scalariform conjugation, the "+" and "-" filaments come into contact, and the cytoplasmic contents of their respective gametes are released. The process of conjugation involves the fusion of the cytoplasm of the "+" and "-" gametes, forming a zygote.

The morphological dissimilarity between the gametes ensures that only gametes of opposite mating types fuse, preventing self-fertilization. This promotes genetic diversity within the population and increases the chances of survival and adaptation to changing environmental conditions.

In conclusion, scalariform conjugation in Spirogyra involves the fusion of morphologically dissimilar gametes. This morphological dissimilarity is crucial for ensuring the successful fusion of compatible gametes and promoting genetic diversity within the population.

B