Final answer:
small seeds like lettuce require light for germination, specifically a) red light. Experiments show red light promotes germination, but far-red light can reverse this effect, influenced by timing relative to the red light exposure.
Step-by-step explanation:
Many small seeds, such as lettuce, require light for germination. Specifically, these seeds are affected by the type of light they receive during the germination process. Experiments have shown that red light promotes germination while green light is not absorbed well by the seed, thus having little to no effect on the germination process. Far-red light also plays a role, and the interaction between red and far-red light can influence germination outcomes.
For example, in an experimental setup where seeds were germinated in the dark, exposure to a pulse of red light can initiate the germination process (as seen with Plate A). However, if that red light pulse is immediately followed by a pulse of far-red light (Plate B), the germination can be inhibited, suggesting that far-red light can reverse the effect of red light. Delaying the far-red light pulse (Plate D) results in germination percentages similar to those seen with just the red light, indicating the effects of the far-red light are time-sensitive.
Therefore, the hypothesis suggested by such experimental results is that red light acts as a trigger for seed germination, while far-red light can counteract this effect depending on timing. This interaction between red and far-red light is known to be mediated by phytochromes, which are light-sensitive proteins in plant cells that play a key role in controlling germination and other developmental processes.