Light plays a key role in seed germination in numerous species. The presence of light can wholly inhibit germination in dark-preferring species, while light-requiring seeds will not germinate without light. Botanical researchers are just beginning to understand the complex pathways by which light, or the absence of light, signals seeds to start growing.
Seeds for some types of plants require light for germination, while others require complete darkness, and most can germinate in either light or dark conditions. Although most seeds which require light for germination are quite small, like lettuce and impatiens, it is not possible to determine which types of seeds require light just by observation. Check the seed propagation instructions on seed packets or in seed catalogs to determine the light requirements for any given type of seed.
Seeds which require light may not germinate at all in darkness under soil. To propagate light-requiring seeds outside, mix them with fine sand, dampen it, then spread it on the desired planting area, or cover the seeds with a thin dusting of vermiculite or fine peat moss to allow light through but help hold the seeds in place. To start light-requiring seeds indoors, simply sprinkle them on the surface of the germination medium.
Light is perceived by plant seeds through various photoreceptors, particularly a special type of plant photoreceptor called a phytochrome. Phytochrome stimulation by the reception of light triggers a complex loop of signals between the phytochrome and plant growth hormones which trigger germination and initial seedling growth.
Different light wavelengths have different effects on seed photoreceptors. A 2006 study by researchers at the University of Minnesota examined the effects of light on germination of a variety of native sedge species (Carex var.) in the hopes of finding a way to encourage their growth in restored wetlands. While two of the eight species examined demonstrated modest germination in darkness, the rest all required significant periods of white or red light wave exposure to trigger germination. Interestingly, the germination signals were negated when the seeds were exposed to far-red light---the close-to-infrared light which filters its way through green canopy leaves. This indicated that if these sedges were not germinated and established before other plants leafed out over them, they would likely not germinate at all.
Seeds vary widely in their dormancy, or resistance to germination. Commercial plant growers often break seed dormancy by application of chemicals like ethephon or gibberelic acid, but certified organic growers are prohibited from treating their seeds with these substances. Researchers at Iowa State University studied the effect of light on germination of seeds of three varieties of Echinacea---an important commercial crop for organic growers selling to the herbal medicine industry. They determined that germination resistance could be overcome by exposing the seeds to light around the clock while in a cold, moist environment. This discovery runs counter to most gardener's intuition to start seeds in a warm, dark atmosphere.