When the leaf length ranges are given in a plant species' description, what do the numbers in parentheses represent?

Anytime you see this being used in terms of a range...

• the numbers outside the parentheses represent the normal or expected range, and

• the numbers inside the parentheses represent atypical extensions beyond the normal range that are uncommon but possible and recorded.

Radford, Ahles, and Bell's 1968 flora, Manual of the Vascular Flora of the Carolinas confirms this in its preface (page vii):

In instances where most measurements for a character fall within one part of the total range of variation, the more infrequent or extreme size range is given in parenthesis as an extra measurement: e.g., "leaves 4-8 (16) cm wide", not "leaves 4-16 cm wide."

So, in your example, you can interpret the first range as follows:

• Blades are typically 4-5 mm long (but sometimes 6 mm long) and typically 1-1.5 mm broad (though sometimes only 0.5 mm broad).

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^{Radford, A.E., Ahles, H.E. and Bell, C.R., 1968. Manual of the vascular flora of the Carolinas. Univ of North Carolina Press.}