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The geography of the spontaneous ligneous flora of the Alps is analysed along a transect of 200 km between Vercors and the upper Pô Valley. Floristic change linked to altitude occurs in a continuous mode, comparable across all the slopes observed, yet not uniform: changes begin above 1,300 m and intensify above 1,800 m, where an altitudinal floristic revolution occurs. Below 1,300 m, the absence of any floristic expression of variations in altitude and the correlative uniformity of vegetation point to the existence of a thermal threshold marking the upper limit of a «phytogeographical domain» of plains and low mountains. Given that a second thermal threshold separates the non-forest landscapes of the high mountains from the forest landscapes of the middle altitudes, three phytogeographical domains stretch along the altitudinal continuum: the first is a floristic domain, the other two are only landscape domains, since the flora there varies continuously with altitude. The variations in flora over the whole transect do not reflect rainfall variations: the vegetation on these middle Western Alps does not show any major hydric stress.