It's often said that the last thing you need to worry about when in a survival situation is what you're going to eat. The human body is highly resilient, and can go without food for longer than you think. Shelter and water, on the other hand, are pretty much non-negotiable in order to survive.
However, it is possible to familiarize yourself with edible wild plants before you get into a survival situation, in which case you'll have a good idea of which wild plants you can eat if necessary. In addition, once you learn to identify some of these common edible wild plants, you might be able to add them to your diet while you're still in the city.
Probably the easiest plant to identify, and one of the most widespread, dandelions (pictured at top) are a great introduction to wild plant foods. The entire dandelion is edible, from the root to the flower, and can be found quite easily in many areas, even in early spring or late fall. The dandelion flower can be eaten straight from the plant, and the taste is sweet and mild. Of the leaves, the young ones tend to taste the best, as they get a little bitter as they mature, but older leaves can be briefly boiled to remove the bitterness. It's also advised that you boil the root before eating, and the water used to cook them can be drunk as a tea afterward.