Lamium amplexicaule produces both cleistogamous (closed) and chasmogamous (open) flowers on one individual during the spring in northern California. A comparative, developmental study throughout the inflorescence documents the change from cleistogamous to chasmogamous flower production in the main shoot. Measurements of floral organ dimensions throughout development were used to compile comparative, allometric growth plots of the organs of flowers from lower to upper nodes of main shoots. Results support Lindman's concept of precocious development to explain the morphology of cleistogamous flowers in L. amplexicaule. Cleistogamous flowers produced at the first flowering node had small anther sacs, narrow corollas, and low pollen counts. Upper-node closed flowers had high pollen counts, large anther sacs, and wide corollas like the chasmogamous flowers but failed to achieve anthesis; they are termed "pseudocleistogamous." The shift from low to high pollen counts from the lower to the upper node flowers, along with the increase in anther sac size and corolla width, was correlated with anthesis. These changes in floral morphology, as a function of inflorescence development, are considered analogous to heteroblastic leaf development, a feature of progressively maturing vegetative shoot systems.
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Elizabeth M. Lord (1979) studied this question.