Abstract
Francis Darwin (1848-1925) was one of the first to study in detail the development and function of stomata. However, since the green leaves of flowering plants are the most conspicuous and suitable organs for the study of cell development and transpiration activity, most of this "Darwinian" research has focused on these sun-exposed organs. In this study, we analyzed the development of stomata along the hypocotyl of 5-day-old light-grown sunflower (Helianthus annuus) seedlings. We document a positive correlation between cell density and DNA content along the axial organ, and the step-by-step development of fully functional stomata in the meristematic region. However, along the stem, from tip to base, the size of the guard cells remained largely constant, whereas the epidermal cells in the neighborhood of the guard cells elongate by 100%, and those farther away by almost 300%. We suggest that symplastic isolation of dumbbell-shaped guard cells is responsible for this lack of growth, and that an inhibitory effect of the guard cells is involved. In addition, we discover and describe epiphytic bacteria on some of the epidermal cells investigated. The role of these microbes, as commensals or symbionts, is unknown.