Here is the deal. Some of which you may not be aware of.
If there is no description meta-tag within the source code, you should add one. It should compliment the title tag and h1 tag as well as being long enough to span more than one line in the SERPs and no longer than 170 characters. Do not push the character limit. The description meta-tag should contain enough semantic clues (keywords) to cover the more important ways people will or do find your page. There should always be a description meta-tag.
Having said that, if there is no description meta-tag for that page, Google, Bing, and others will find a portion of content to use for the SERP snippet. You can control how the SERP snippet appears most of the time with a well crafted description meta-tag, however, the SERP snippet can again be a portion of content depending upon the search query. This is why I say that the description meta-tag should have enough semantic clues (keywords) to cover the most important search queries.
For the record, for Google, the description meta-tag is strong clue for how your page should be found. It is not content therefore is not really weighed, but is part of the blended result sets that make up the SERP results making it one of the most important HTML elements. It can be ignored if it does not match the other strong clues such as the title tag and h1 tag. This means that the description meta-tag should not be manipulated for search. It has to semantically correlate (be similar in linguistics/language) with the other tags. None of these tags should be identical or even too close, just similar.
