Large, empty colorless cells of the adaxial epidermis along the veins of grass leaves are:

1. Lenticels

2. Guard cells

3. Bundle sheath cells

4. Bulliform cells

Grass leaves curl inwards during very dry weather. Select the most appropriate reason from the following :

1. Tyloses in vessels

2. Closure of stomata 

3. Flaccidity of bulliform cells

4. Shrinkage of air spaces in spongy mesophyll