Final answer:
Rigid plastic sheets in greenhouses trap solar radiation for warmth and protect plants by maintaining consistent internal temperatures.
Step-by-step explanation:
There are two significant advantages of using rigid plastic sheets for covering greenhouses. First, rigid plastic sheets are excellent at trapping solar radiation, similar to glass. This trapped energy raises the temperature inside the greenhouse, creating an ideal environment for plant growth.
Second, because these sheets are sturdy and protective, they help to maintain a consistent temperature within the greenhouse despite external temperature fluctuations. This stability is vital for the plants which often have rigid cell walls requiring protection from changes in temperature.