Over the past few months, some type of grass-like weed (?) has invaded a region of my lawn. This area receives full sun. The weed grows in small, very thick patches. The patches of grass are about 1 sq ft in size, some larger, some smaller. The patches can be spaced anywhere from a few inches apart to a few feet.

It appears like normal thin blades of grass, except for the tiny green balls that grow on some of its blades. I assume these are seeds. These green balls look prickly, but they are somewhat soft to the touch.

Here are some additional pictures I took:

http://i.imgur.com/mN7y1.jpg

http://i.imgur.com/EtNod.jpg

http://i.imgur.com/mbonYh.jpg

http://i.imgur.com/eihZth.jpg

I have tried searching for similar photos of this weed, but all I've been coming across are irrelevant results, or similar looking seeds on completely different plants. I'm hoping someone here can provide me with an answer of what it is, and what I can do to treat it.

Thanks.

Instead it's a nasty weed called nutgraas

See this
http://www.sunset.com/garden/garden-basics/how-to-control-weeds-004000 00043681/page6.html

I assume the small green balls are actually the flower? Mine haven't bloomed yet, but this picture I found on Flickr seems to be the closest to what I have: http://www.flickr.com/photos/97769244@N00/1347478544/

Also, shortly after I posted, I found another plant called Sparganium which bears a striking resemblance to the current appearance of my yard. Here's a pic: http://www.pwrc.usgs.gov/history/herbarium/sparganium_americanum.jpg
However, it appears to be an aquatic plant, commonly found in wet areas. I have no idea how it could have gotten into my yard, or how it would even survive in my dry, full-sun soil.