EVOLUTION OF LEAF ARCHITECTURE IN THE CHAPARRAL SPECIES FREMONTODENDRON CALIFORNICUM SSP. CALIFORNICUM (STERCULIACEAE)

Abstract

The chaparral species Fremontodendron californicum ssp. californicum and the cloud forest species Cheirostemon platanoides arose from a common mesophytic stock. The leaf of C. platanoides is proposed as a model for the ancestral leaf and provides an architectural baseline allowing identification of the types and relative magnitudes of morphological and anatomical modifications which have occurred in the leaf of F. californicum. The leaf of F. californicum reflects intense selection for reduced surface area; however, both deletion and addition of lobes has occurred. Vein density has increased due to a greater number of more highly branched terminal veinlets. Mechanical structures, i.e., sclerenchyma, cuticle, show a quantitative reduction; whereas, water storage tissue, i.e., mucilage cells, has been augmented at the expense of spongy mesophyll. The leaf has become weakly isolateral and the number of palisade strata has increased. No increase in the volume of palisade tissue per unit volume of mesophyll is apparent due to a reduction in palisade cell length and palisade packing density. Total leaf thickness has not increased. The indumentum shows selection for non‐living, pedestalled stellate trichomes which form a transpiration‐limiting canopy above the stomata. Stomatal density has decreased while guard cell dimensions have become larger. Some structural modifications in F. californicum appear to have evolved directly in response to long‐term water stress, others seem more directly related to leaf diminution. Xeroplastic changes in foliar structure are in some cases unreliable indicators of ultimate xeromorphic modifications arising in an evolutionary context.