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My textbook says "As the seed matures it loses water and enter dormancy". I am confused as to whether this happens before the seeds are transported or after, and as to how seeds know to become dormant when required and then become active again.

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I presume what the author is trying to convey is that as a seed ages, it enters a quiescent phase as a result of natural desiccation. Without more context, I'd guess that the quote you've provided here is referring to secondary dormancy, which is a response to unfavorable germination conditions. By definition, secondary dormancy occurs after the seed is released from the plant.

The mechanisms of secondary dormancy are not well understood, and resumption of embryonic function could be dependent on any number of factors. Generally, dormancy continues until such time as favorable conditions are observed by the dormant seed. For Avena sativa, seeds become dormant if temperatures in excess of the maximum for germination are observed. Other seeds become dormant in response to over- or dehydration, temperature variability, age, other factors, and combinations ad nauseum.

Some of the more common environmental causes of exiting dormancy include afterripening, chilling and lighting.

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So as a seed matures it becomes dormant, and is than released and transported. Eventually once it ends up in a favorable habitat it becomes active again –  user3034084 Dec 30 '13 at 20:46
    
It happens that way in primary dormancy. What I described above was secondary dormancy, in which release and (potentially) transportation occur prior to dormancy. Primary dormancy usually ends the in a similar manner as with secondary dormancy, but the seed is released and transported in an already-dormant state. Reading over the textbook quote again, it could well have been referring to primary dormancy. –  Chris Cashwell Dec 30 '13 at 20:54
    
I'd just like to say how happy it makes me to see primary and secondary dormancy being discussed on bio.SE: very happy indeed :). Also: this is exactly the answer I would have given. –  Richard Smith Dec 31 '13 at 0:47
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