It has been my experience that a plant with a very well-developed root system that's given plenty of water will be the most heat-resistant. I have one plant (Nirvana Big Bud) in ten gallons of dirt right now, and even though the temperatures are often well over 100 degrees, she's shown no signs of stress. My guess is that she keeps cool the same way us people do: by sweating. And like us, she needs to drink a lot. Whenever two leaves are against each other, there's always lots of condensation between them, so I know that she's taking in all that water I dump in her pot.

This has only been my experience, although I've heard evidence from other growers that this is the case. It would still be important to have good ventilation so that all that water vapor goes somewhere.