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The figure to the left shows a cumulative plot of the number of taxa against the number of squares in which they were recorded. All 662 taxa were found (obviously) in at least one square, 530 in at least two, and so on. No plant was found in every square, not surprising as some squares near the Humber are very small and of unusual habitat type. The most frequently encountered plant was Cirsium arvense which was recorded in all but one of the squares.
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The "essential" flora of Hull seems to consist of about 224 plants, each of these can be found in 20 or more of the 119 squares surveyed. The figure is somewhat arbitary, but has been arrived at by examining the above diagram. The graph shows a marked change in slope at this point, and the steady increase in the number of species as the number of squares decreases, becomes more marked. This change is interpreted as the point at which the more exotic elements, with very specific habitat requirements, become significant.
