Preheat oven to 350° F. Place the pita rounds on a baking sheet and toast in oven until crisp and light brown, about 10 minutes. Break into bite-size pieces and set aside. Toss together the cucumber, tomatoes, scallions, fennel, celery, mint, and parsley. Cover and chill. Meanwhile, in a small bowl, whisk together the oil, lemon juice, garlic, salt, and pepper. Combine the vegetables and dressing, and fold in the pits pieces. Serve with hummus, if desired.