Question

Grass leaves curl inwards during very dry weather. Select the most appropriate reason from the following.
A) Closure of stomata
B) Flaccidity of bulliform cells
C) Shrinkage of air spaces in the spongy mesophyll
D) Tyloses in vessels

Answer 1

Hint: In most of the dicot leaves, stomata are present more on the lower leaves while there is less on the upper surface, while in monocot leaves (grasses), stomata are distributed equally on both the surface, so there is more chance of transpiration from its upper surface, so in order to protect itself from wilting they undergo a special mechanism.

Complete answer:
Bulliform cells or motor cells are special kinds of cells present in isobilateral leaves. They are comparatively larger cells than other epidermal cells. These cells are hygroscopic in nature and contain large water vacuoles which store water and if necessary, they can remove that water. They play a very important role in leaf folding and unfolding. They also prevent the loss of water in a stress condition. If the water in the atmosphere is abundant, then due to their hygroscopic nature, they absorb water and store them in their vacuoles and become turgid. When there is a stress condition (dry condition), in which water content is very less in the atmosphere, then the water stored in the vacuoles are released out and the motor cell becomes flaccid. Due to this, folding of leaves occurs to prevent the loss of water from the vacuoles. E.g. grass leaves curl inwards during very dry weather due to flaccidity of bulliform cells.
Hence, the correct answer is option B.

Note: According to the distribution of stomata, plants are of the following types:
a) Apple and Mulberry type: in such plants, stomata are present on the under surface only.
b) Potato type: More stomata on the lower surface than on the upper surface.
c) Oat type: Stomata are equally distributed on both surfaces.