Transpiration

The question asks why grass leaves curl inwards during very dry weather. Let's understand this phenomenon by examining the structure of grass leaves and the role of bulliform cells.

Explanation

Grass leaves have specialized cells called bulliform cells on their epidermis, typically on the upper side. These cells play a crucial role in helping the leaf manage water loss during dry conditions.

1. Function of Bulliform Cells:
   • Bulliform cells are large, bubble-shaped cells that can lose or gain water, affecting the turgidity of the cells.
   • During dry conditions, these cells lose water and become flaccid (lose turgidity), causing the leaf blade to curl inwards.
   • This inward curling minimizes surface area exposed to the harsh environmental conditions, reducing water loss through transpiration.
2. Process During Dry Weather:
   • In dry weather, water scarcity leads to a loss of turgidity in bulliform cells.
   • This flaccidity induces the curling of grass leaves as a survival mechanism to conserve water.

Justification of the Correct Option

The correct answer is Flaccidity of bulliform cells. This is because:

• Closure of stomata is a response to water loss minimizing transpiration but does not directly cause leaf curling.
• Shrinkage of air spaces in spongy mesophyll contributes to the overall reduction in leaf size but not specifically curling motion.
• Tyloses in vessels are related to the blockage of xylem in some plants, which is irrelevant to the curling mechanism during dry weather.

Thus, the leaf curling is predominantly linked to the flaccidity of bulliform cells.