It looks almost as if the leaves are burned. I know we have had plenty of bright sunshine recently, and more-or-less constant strong breezes, but I don't think it has been that hot. I think our maximum daytime temperature must have been about 20 or 21C. And I have been very good about watering.
I have another theory. Could it be acid burn? I mean acid in the sense of Fox urine? Maybe this plant has become a territorial marker for the nocturnal animals. Do you think this is possible / likely?
Just to give you something a bit nicer to look at in this post, here are a few pictures of some young lettuces:
|Marvel of Four Seasons|
|Pancalieri (an Endive, not a Lettuce)|