Effects of Exposure to Direct Sunlight Upon the Development of Leaf Structure of Two Deciduous Shrub Species

The present anatomical study of sun and shade leaves of two shrub species Menziesia glabella and Lonicera glaucescens is an outgrowth of interest in the response of ground cover plants to increased sunlight as the result of logging operations. Leaves of both shrub species developed on plants fully exposed to the sun are smaller, thicker, more compact, with longer palisade cells, less spongy mesophyll, and thicker cuticle than comparable leaves from plants growing normally in deep shade. Evidence is given for the views that (1) differences in leaf expression are not predetermined by the environment of the bud during its formation the previous season, and (2) structural modifications that result when typical shade buds are suddenly exposed to full sunlight are merely an indication of the plasticity of leaves to light.