And I have to go away for two days. Since I don't own a system to turn the lights on and off automatically, what is my best option: two days constant light, or two days constant dark? What are the pros and cons of each?

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YARGLES!

constant light. there are no pros of constant dark so early in its life

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Here, throw this record on, pack a bowl, take a hit.
www.zeitgeistmovie.com iGrow

yea just leave the lights on, but leave them far enough away that the seedlings wont grow and touch the lights.
They grow fast at this age
Well mine did

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yah those ideas sound like they should be rite.. becuz i dun think ur plant will grow in 42 hours of constant darkness and make sure u water it b4 u leave

That's what I thought. The more phtotsynthesis the better! I'm thinking I'll also give them an extra-large watering (relatively speaking) to hold them over while I'm gone. And you're right, they are growing fast: its been three days since they sprouted, and two have already hit 7cm.
You mentioned that there's no benefit to constant dark "so early in its life". Is there a benefit to it later?

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YARGLES!