Growing Lettuce in Extreme Heat

Despite countless gardening defeats this season (i.e. the wilted cucumber vine in the photo), there have been some exciting successes.  

For example,  notice the lettuce growing at left in the photo.  This particular variety, like most lettuce varieties shouldn't really be able to grow right now in this extreme heat.  But, it's going strong!  What's the secret?

The leaning vine-covered trellis on its south side provides the tender lettuce good shade during the hottest parts of the day.  

Since this pic was taken, the cucumber vine has shriveled up entirely.  I don't plan to uproot it, though, until the temperatures cool enough for the lettuce to survive without the shade that wilted vine is offering.  Talk about multi-purposing -- that cucumber vine is still working for me, long after it's produced its last fruit.