Water loss occurs by diffusion, the rate of which is determined by Fick's law.

rate = (surface area x difference in concentration) ÷ thickness of exchange surfaces

How does trapping a layer of moist air reduce the rate of diffusion?

Want to more about Sorghum? Follow this link (P.S. you really ought to get out more).

Sorghum

The text below is adapted from material supplied by AQA.

Sorghum forms the staple diet of many of the people living in the drier parts of Africa and central India. Sorghum is one of the few cultivated plants that are able to grow in hot conditions and survive long, periods without rain. Because of this, it is a very important crop in the drier parts of tropical Africa and central India. Sorghum has various features which enables it to survive.

1. It is very efficient at extracting what little water there is from the soil and has an extensive root system. This extends outwards for approximately 60 cm from the plant and down to around 80 cm.

2. It shows adaptations which reduce the rate at which water is lost from the leaves by transpiration. In particular the leaves have large thin-walled cells on their lower surface. These lose water rapidly in dry conditions with the result that the leaf curls up and traps a layer of moist air. The leaves also have a thick wax cuticle and a reduced number of stomata.

3. In very dry conditions, sorghum stops growing. It recommences growth following rainfall.

Plants adapted to dry conditions are called xerophytes.

Another plant adapted to dry conditions is marram grass, normally found on sand dunes. Look at the picture of a section through a marram leaf. What adaptations to dry conditions can you see? Click on the picture to see if you are right.