Stomata contain pores that regulate gas exchange through the shoot epidermis. The control of stomatal formation is central to leaf development and plant productivity. Stomata are produced and patterned via asymmetric divisions. TOO MANY MOUTHS (TMM) is a leucine-rich repeat receptor-like protein that regulates stomatal number and distribution. TMM is expressed in a population of stem cells during leaf development. It appears to control the balance between cell proliferation and differentiation in a cell-type- and organ-specific manner. TMM is also likely to receive spatial cues used to orient the plane of asymmetric division so that stomata do not form adjacent to each other. Thus TMM probably functions at multiple levels of a developmental hierarchy. Downstream signaling is likely to require association with other proteins because TMM lacks a cytoplasmic domain. The TMM protein complex will be analyzed biochemically, and the identity and function of interacting proteins will be characterized. The relationship between TMM and another gene with overlapping functions will be defined at the levels of genetics, expression, biochemistry, and development. The expression pattern of TMM will be evaluated in the context of leaf and shoot development. Asymmetric divisions will be studied to determine whether TMM orients an axis of polarity and how it controls the division plane. This work addresses fundamental questions in plant development such as how a receptor regulates the plane and frequency of asymmetric division, and how cell position and communication contribute to pattern formation. Because other plants, including crops, have TMM-like genes, this work should provide a context for a wider understanding stomatal development and of the evolution of a key feature of land plants.