Gardening is full of surprises. This flower is what you get if you let arugula go nuts, or bolt. Dr. Punger at Permission to Mother asked me for more information on bolting plants the last time I did a garden update and I thought other people might be interested as well.
I looked in all my general gardening information books and was surprised to see only two mentions of bolting and no really good explanation. Basically, bolting is gardener lingo for explaining that certain plants are going to flower and seed. This term is reserved for those plants which we grow primarily for the leaves and not the fruit or seeds. So tomatoes don't bolt, they "flower" and "set fruit" - happy, complementary terms. Lettuces, dark leafy greens, and herbs bolt - a harsh, bitter term. For these plants we endeavor to put off their going to seed as long as possible for our benefit.
I seeded several square feet with mixed lettuce seed (meant to be eaten young) but I got one solitary plant out of it (damn Japanese beetle grubs). It actually took quite some time to bolt, and here it is, just prior to actually flowering. Most greens get pretty bitter once they start to bolt, becoming much less appetizing.
There are a number of factors that cause a plant to bolt, but they all come down to stress. Most leafy greens are cool-weather plants and bolt when it gets too warm for their liking. You can get around this to a point in a couple of ways. Look for bolt-resistant varieties (some of these are hybrids, which you may want to keep in mind if you avoid them as I do), keep the plants well watered (but don't drown them!), and try planting in a shady area or on the north side of a building if you want to grow them during warmer months. If you have the space I imagine you could also grow them inside, like in a basement, with grow lights if you really want mid-summer homegrown lettuce in a hot climate. Some greens do not transplant well and the shock of transplanting can also cause them to bolt. This is true of cilantro, which is easy enough to start from seed. If you really want a continuous supply of cilantro you may want to consider succession planting, or starting a new set of seeds every two or three weeks, so that you always have fresh cilantro.
Remember that mysterious giant plant with yellow flowers? Well I think I determined it was kale, although not a kale I recognize. I ripped it out in a fit last week and look at the size of the thing. I am 5'3 and it was bigger than me (and heavy!). Amazing.