Here are some photos of various leafy greens going to seed:
This is some lettuce seed in the stage after it has flowered (above)
When collecting leafy green seeds, pay attention to the plants. They will begin to "bolt", grow upwards & create flowers (they will no longer taste very good at this stage), let those flowers bloom & die back, and then seed pods will form. Let them brown & then collect! Dry further indoors, pop open pods, and there is the seed!
Romaine Lettuce bolting, it is about to send out it's flower stock
Romaine sending out flowers
Leafy green flower stocks against a beautiful august evening sky
Red Romaine just about to flower
After the flowers have bloomed, you will have to wait another few weeks for the pods to form and start to dry out . . .watch carefully, when they are ready, they may burst open & you will loose the seed. Take them inside to dry before that stage . . .