The pattern of peltate glandular trichome initiation and ontogeny on expanding peppermint (Mentha x piperita) leaves was defined by surveying the populations of peltate glands in each of seven developmental stages within sampling areas of leaf apical, mid-, and basal zones for both abaxial and adaxial surfaces. It was shown that new peltate glands continue to form until leaf expansion ceases and that regions of active gland initiation are unevenly distributed. The distribution of gland initiation reflects the basipetal pattern of leaf maturation, with relatively immature regions at the leaf base continuing to produce oil glands long after gland production has stopped at the leaf apex. The proportion of glands in the secretory stage as a function of leaf development and the direct observations of living glands over a period of 33 h indicate that a period of only 20 to 30 h of secretory activity is required for filling of the gland storage compartment with essential oil. These findings are discussed in relation to earlier literature describing age-related changes in glandular essential oil content.