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Published by the American Geophysical Union as part of the Special Publications Series. This is a research monograph and not a textbook. Here I demonstrate analytically how the observed, opposing, latitudinal gradients in the average range and richness of local vascular land plant species are (outside the moist-tropical zone, at least) driven primarily by the local temporal and spatial variability of shortwave radiative flux at the canopy top. (The term'richness'as used here means the local number of different vascular land plant species unlimited by the size of the area sampled.) The hypotheses are simplistic but are nevertheless convincingly accurate in extratropical latitudes when tested against observations over the continental land surfaces of the Northern Hemisphere, the only areas tested here.