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

You would need as large a sample of seeds as was convenient for counting, say 30. The seeds would be provided 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 allow germination.   

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

The similar number of seeds, from the same batch (same packet), would be given identical    conditions of moisture and temperature but placed in daylight for the same period of time. At the end of this period the numbers of seeds which germinated in every case would be counted and compared.