response: Well I can tell you that the bitter taste is especially found on the ribs of the outer leaves, which are older.
original_response: Yes that is it. You should pay attention to the  thick ribs, especially on the older outer leaves
history: I do not care for romaine lettuce at all. Oh?  Did you know it's tolerant of heat though, unlike most lettuces? I just have never cared for romaine lettuce. Something about the taste. I was not aware it was tolerant of heat. Do you know anything else interesting about it? Well, you know that fine-bitter herb flavour it has?  It's because of the milky fluid in the thick ribs. I was not aware that is where it got the bitter taste but I think that is what I do not like. I do know that it is the usual lettuce used in Caesar salad. That is about all I know about romaine lettuce.
knowledge: The thick ribs, especially on the older outer leaves, should have a milky fluid which gives the romaine the typically fine-bitter herb taste.