Possible Clues:

• Cos and Iceberg, say
• Little gems from rough-cut steel
• Iceberg and romaine, e.g
• Not greens for putting on but for taking in
• Green leaves for salads
• Romaine and iceberg
• Iceberg and romaine
• Romaine and iceberg, e.g.
• Heads for the supermarket
• Green leaves for salad