Summary
In this initial paper of a series on physio-ecological parameters in leaves, some basic aspects of foliar morphology and anatomy are considered in relation to environmental conditions, the main evolutionary stages that gave rise to leaves are analized. The basic morphology and anatomy of leaves is traced throughout the evolutionary history of plants, from a postulated Rhynia-like ancestor to the more recent (in an evolutionary sense) types of leaves. Some of the problems and selective pressutes that might help elucidate and interpret leaves from a physio-ecological stand point are analized. Also considered is the possible influence, in view of current data, that leaves might have on the structure of the stele. Some of the basic factors that might excert selective pressure, such as temperature, light and water are also considered.