Flower:

An elongating raceme, with blooming flowers in a small flat, compact cluster at the top of the stem. Flowers are 1/8 to ¼ inch long with 4 white or creamy colored petals, 4 green to yellowish sepals darker on the tip end, several yellow stamens, and a flower stalk up to ½ inch long. The petals may be spreading but they're often seen only poking out of the sepals by a little bit and not fully open. A plant typically has multiple flowering branches.

Leaves and stem:

Stem leaves are up to 3 inches long and 1 inch wide, generally lance-like, toothless, mostly hairless except perhaps around the edges, with a pointed tip and a pair of lobes at the base that wrap around and clasp the stem. Leaves become progressively smaller as they ascend the stem, and are covered with a waxy powder that gives them a blue-green cast.

Basal leaves are more spatula shaped with a blunt tip and shallow lobes or largish irregular teeth, hairy to varying degrees, especially on the underside along the midrib and along the edges. The stem is mostly smooth and light green, often also with a bluish cast.

Fruit:

Fruit is a straight slender pod up to 2½ inches long, containing 2 rows of tiny seeds. Pods are erect, crowded and hug the stem.

Notes:

Tower Mustard is more commonly known as Arabis glabra, but recent DNA testing has moved it to genus Turritis. It is not likely to be mistaken for any other species, easily identified by the clasping leaves and erect pods close to the stem. Most references put its maximum height at about 3½ feet, but I've seen late season plants close to 5 feet tall.