Obviously, this is the time of year to post some good fall foliage photos. More and more, I am also considering honey locust (Gleditsia triacanthos var. inermis) to be in the underappreciated category for autumn color. Right now, these trees -- so common in NYC -- are at various states of undress. Some have full canopies of green leaves, others have begun to turn a burnt-gold color, and others have completely defoliated.
Gleditsia is unusual for it's heterophyllic foliage. At times, the plant has a basic, pinnately compound leaf. That means that the leaf consists of smaller leaflets, all arranged on a straight line (horsechestnut, on the other hand, has a palmately compound leaf; leaflets are arranged like fingers from a palm). In addition to the pinnately compound leaves, honey locust can also have bipinnately compound leaves -- that means that the individual leaflets are compound and are composed of sub-leaflets. You can see both leaf types in the photo below; while the branch in the center of the photo has mostly pinnately compound leaves, the branches to the right and left have bipinnate leaves.
The trees below - sitting next to the Hudson - have lost all their leaves. It's a good chance to see the winter habit.
To me, this is a very recognizable tree in the winter -- the branches are knotty and have a zig-zaggy habit. The bark is also quite dark and will occasionally peel in thick plates.
Gleditisia's genus is named after a German botanist. Triacanthos means three-spined and conversely, inermis means thornless. The botanical name makes more sense when you realize that the straight species of honey locust is very thorny. You can occasionally see thorned honey locusts in the city -- in Central Park there are a few near the statue of the Polish King Jagiello at the east side of the Turtle Pond -- but most of the time these trees will be thornless.