How would you design, in principle, an experiment to test the hypothesis that a particular variety of lettuce seed required daylight in order to germinate

You would require as large a sample of seeds as was convenient for counting, say 30. The seeds would be given with water and a suitable temperature. The seeds, in a suitable container, would be placed in a light-proof box or cupboard for a period long enough to permit germination.   

The box or cupboard would not be opened during this period.

The similar number of seeds, from the similar batch (same packet), would be given identical   conditions of moisture and temperature but placed in daylight for the similar period of time.

At the end of this period the numbers of seeds which germinated in every case would be counted and compared.