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Definition of "lobe":
(reproduced from Royer, D.L., Wilf, P., Janesko, D.A., Kowalski, E.A., and Dilcher, D.L. (2005) Correlations of digital leaf physiognomy to climatic and ecological variables: potential proxies for the fossil record. American Journal of Botany 92(7): 1141–1151.)
Find the axis of symmetry of the indentation, and project a line (d) along this axis from the apex of the indentation to the midvein. Typically, the axis follows the trend of the feeder vein for the indentation. Project a perpendicular (p) from the apical sinus of the indentation to d. If the distance from the apex to p is greater than 0.25d, the feature is a lobe.
It is possible that this definition may lead to some large indentations being selected as teeth, for example leaves with compound lobes (e.g., species of Crataegus).
Lobe vs. tooth rule - Fig S1 from Royer et al. 2005. Quercus rubra leaf illustrating the lobe vs. tooth rule. Line segments p and d are defined above in text. Scale bars = 1 cm. Image