From On Food and Cooking by Harold McGee:
“Unlike onions and garlic, leeks don’t form useful storage bulbs, and are grown instead for their scallion-like mass of fresh leaves. (…..) The upper green portion of each leek leaf is edible, but tends to be tougher and to have less onion, more cabbage-like flavor than the lower white portion. It’s also rich in long-chain carbohydrates that give the cooked vegetable a slippery texture, will gel when chilled, and can lend body to soups and stews.”