The grape vine is far more tolerant of cold than the olive, but to produce tolerable wine it demands, at the season of ripening, a degree of heat not much less than that needed by the more delicate tree. These conditions are satisfied in the deeper valleys of the Alps, even in the interior of the chain, and up to a considerable height on slopes exposed to the Sun. The protection afforded by winter snow enables the plant to resist severe and prolonged frosts that would be fatal in more exposed situations. Many wild plants characteristic of the warmer parts of middle Europe are seen to flourish along with the vine. A mean summer temperature of at least 20C (68°F) is considered necessary to produce tolerable wine, but in ordinary seasons this is much exceeded in many of the great valleys of the Alps.