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Is light required for germination of every seed? If not, why don't some plants require it? Is there any difference in the quality and duration of light required ?

It would be interesting if someone could give both physiological and evolutionary reasons.

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Given that germination often happens underground, the answer seems trivially “no”, doesn’t it? –  Konrad Rudolph Dec 21 '13 at 16:18
    
@konrad My first thoughts were the same ! but then I came across an experiment in which they had exposed seeds to different wavelenghts of light and they got difference in no of germinated seedlings. –  biogirl Dec 21 '13 at 16:24
    
The riddle can be solved by realising that different plants have different requirements, and that some requirements are “soft” – i.e. not absolutely necessary but nevertheless beneficial. I’ll let somebody else with more knowledge provide a more in-depth answer – my comment wasn’t meant to imply that this is a bad question. –  Konrad Rudolph Dec 21 '13 at 16:28
    
Botany was long ago, but if I remember correct, the germination happens usually underground as the seeds contain enough material to supply the developing plant for a while. When it comes through the surface it usually needs light to do photosynthesis to grow further. Most seedlings orientate towards the light. –  Chris Dec 21 '13 at 17:15
    
The short answer is that many species do require light for germination (either to initiate germination or to break dormancy). Some require light of a specific wavelength, and some don't require light. Still others actually require absolute darkness, or the absence of specific wavelengths. –  Richard Smith Jan 4 at 2:17

1 Answer 1

Here's a partial answer, based more on common-sense then in-depth knowledge:

Physiological. One reason light is not required immediately is that many seeds are in essence bundles of stored energy (as carbohydrates, I suppose). This is what makes them attractive food for birds, squirrels, humans, etc. When they're able to escape that predation, the growing plant can use that stored energy, rather than relying only on sunlight.

Evolutionary. Seed dormancy can be a powerful tool for fitness – seeds that can remain dormant and germinate during favorable conditions could have a much higher survival rate. These conditions vary from simple: "spring" to more complex: "after having passed through the digestive system of a certain animal"; "after a forest fire"; "when there's a gap in canopy cover / less competition for solar energy". The cues that seeds "read" in order to try to break dormancy and germinate during these times therefore vary among species. Within species, there are probably often a number of cues, and as @Konrad Rudolph suggests these are probably all relatively "soft", or at least act in complex interaction with one another.

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