Fig. 1.
Top row: images of chlorophyll fluorescence from a leaf of Xanthium strumarium following a sudden decrease in ambient humidity at time zero. The image represents a leaf area of approximately 4 cm2. Brighter regions represent portions of the leaf with lower photosynthesis and therefore lower stomatal conductance. Conductance patches appear within the first hour and move about the leaf for several hours thereafter. Bottom row: pixels that tend to brighten over a period of a few minutes are coloured red, whereas those that tend to dim are coloured yellow. Pixels showing little change are black. These coloured structures propagate coherently over the leaf surface. The movement of the patches is best seen in the supplementary video clips available online at http://aob.oxfordjournals.org.