This document provides instructions for determining leaf constants of Datura and Vinca leaves through quantitative microscopy. It describes how to calculate the vein islet number and vein termination number of Datura leaves, as well as the stomatal number and stomatal index of the lower epidermis of Vinca leaves. The procedures involve surface preparation of leaf pieces, examination under the microscope using a camera lucida to count structural features within delineated areas, and calculations to determine the constants based on the counts within a 1 mm2 area.