Guard cells help in

What causes the opening and closing of guard cells of stomata during transpiration?

Early in the morning,excess of water collects in the form of droplets near the tip of leaves of herbaceous plants,this is due to

In guard cells,when sugar is converted into starch,the stomatal pore

The conditions under which transpiration would be most rapid are:

$(i)$ The ...$a$... help in maintaining the pressure gradient in the sieve tubes.
$(ii)$ Stomata regulate the process of ....$b$..... and gaseous exchange.