Tough as a weed!

Why doesn't weed-killer herbicides kill lawn grass too? How could grass be more resistant to the chemicals than weeds?

Just curious -- the question arose while watching a TV commercial advertising weed killer that is harmless to grass.

Broadleaf weed killers are selective herbicides that work in lawns by targeting dicots that are actively growing while not harming the monocots such as lawn grass. The most common type of broadleaf weed killer is 2,4-D also known as Trimec.
2,4-D is an auxin or plant hormone that, when applied to the leaf area, simply confuses the plant to death. Since auxin is the hormone that makes plants grow, when it's sprayed over the entire plant, the plant doesn't know which way to grow, becomes deformed, then dies.

In monocot stems, the vascular tissues " the phloem and xylem " are in bundles scattered throughout the stem, and they typically lack a vascular cambium. In dicot stems, the phloem and xylem are in rings around each other. They nearly always have cambium. Despite a pine's stem structure resemblance to dicots, it is neither a dicot nor a monocot. A pine is a conifer, which is not a flowering plant at all.

Partly as a consequence of the arrangement of the vascular tissue, in monocots, there is very little new phloem and xylem added to the stem. Thus, monocot stems do not grow significantly thicker each year. Any change in thickness is due to the cells getting slightly larger. On the other hand, dicot stems can add new vascular tissue and thus grow thicker with time. Most flowering trees are dicots.

Mechanism of herbicide action

2,4-D is a synthetic auxin, which is a class of plant growth regulators. It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues causing stem curl-over, leaf withering, and eventual plant death. 2,4-D is typically applied as an amine salt, but more potent ester versions exist as well.