Optimal Leaf Form

Abstract

The size of leaves typical for specific climates has been studied for many years, and several investigators have considered the" leaf size class" as an indicator of climatic conditions (Raunkiaer, 1934). Bailey and Sinnott (1916) concluded that the form and size of leaves were more a result of environment than of genetic history, although the latter was certainly an influence. Benson et al. (1967) reported ecotypic differentiation of leaf form with respect to slope exposure for a hybrid population of Quercus douglassii x Q. turbinella. They suggested that hybrid variability may permit the rapid evolutionary selection of characters best suited for the particular microclimate. They reported that individuals found on the northeast slope had leaves of significantly larger dimension than did those growing on the more arid southwest slope (Fig. 5.1). It is generally considered that the reduction of leaf size in arid areas has the effect of conserving water, but quantitative evidence of the effects of leaf size has been available only recently. The effects of leaf size are inseparably coupled with other characteristics of the leaf and the environment. Proper evaluation of the significance of any characteristic must consider all environmental and biological factors. The analysis must include the primary meteorological and edaphic parameters; solar and thermal radiation, air temperature, atmospheric vapor pressure, air speed, atmospheric gas concentra- tions (C02, O2), and availability of soil moisture. Biological parameters include absorptivity to radiation; stomatal and mesophyll resistance to uptake or loss of carbon dioxide, oxygen, water vapor, and other gases; size, shape, and orientation of the organism; and the temperature range critical to survival. When the biological and environmental parameters are known, one can properly evaluate the biological responses to the environment and thereby determine the significance of variations in the individual characteristics or parameters. The biological responses considered in this paper are leaf temperature, transpiration rate, net photosynthesis, and the ratio of photosynthesis to transpiration.