Thus lettuce seeds germinate only when placed in white or red light. Buried in deep soil, they will not germinate. Given that lettuce has a small seed, I think you can figure out why evolution arrived at this solution. This form is biologically inactive or inhibits responses. Thus if lettuce seeds are placed in far-red light they do not germinate.
Large seeds have lots of storage material.
If their seed coat is very thin, their evolution may have arrived at a completely different response. Think about pea seeds. They are large and have very thin seed coats. If a seed's embryo is not completely developed, some additional maturation may be needed before the seed can sprout. Examples include orchid seeds.