David F. Parkhurst

THE ADAPTIVE SIGNIFICANCE OF STOMATAL OCCURRENCE ON ONE OR BOTH SURFACES OF LEAVES

SUMMARY (1) The relationship of stomatal occurrence (on one or both sides of leaves to environment was studied by literature search, mathematical modelling and investigation of herbarium specimens. The starting hypothesis, that hypostomatous leaves (with stomata only on the underside) should occur in dry habitats, was contradicted by all three sources. (2) The models, based on mass-transfer physics, produced three major results; (a) mesophyll thickness is the chief variable determining stomatal distribution, with thick leaves tending to be amphistomatous (with pores on both sides); (b) stomatal distribution does not depend much on environmental variables, but the dependence is strongest under conditions of low water stress; (c) within the limits of the models, amphistomatous leaves appeared nearly always to be better adapted than hypostomatous leaves, indicating that the models are not yet complete. (3) Leaf thickness and stomatal distribution were determined from herbarium specimens of the Indiana species of Compositae, Liliaceae, Salicaceae and Scrophulariaceae. These measurements were then compared with a simple habitat rating derived for each species from published habitat descriptions. Hypostomatous leaves occurred least often in xeric habitats, most often in mesic ones, and again less often in hydric habitats. However, the data suggest that this relationship is secondary, with leaf thickness being the intervening variable. (4) Finally, explanations are considered for the predominance of hypostomatous leaves over hyperstomatous ones.

A three-dimensional model for CO2 uptake by continuously distributed mesophyll in leaves.

Abstract Photosynthetic CO 2 uptake in leaf mesophyll can be modelled in a general way using a second-order partial differential equation, which is derived and discussed and compared with existing models using finite series-parallel combinations of a few resistance elements. When mesophyll porosity is constant and when local CO 2 uptake is proportional to intercellular CO 2 concentration C, the equation takes the form Δ 2 C = α 2 C . Analytic solutions are given for this special case, with the boundary condition being equivalence of internal CO 2 uptake with that moving through the stomates. These solutions show the importance of accounting for the three-dimensional nature of CO 2 transport in the mesophyll, which most existing models do not do. The model is useful for exploring optimal geometry of leaves. Its implications for leaf thickness, stomatal size and spacing, and mesophyll porosity are discussed. In particular, it yields a prediction of optimal leaf thickness very close to values commonly occurring in nature.

Graphical Methods for Exploratory Analysis of Complex Data Sets

ABSTRACT In scientific investigations, performing initial exploratory studies can help in generating hypotheses and deciding what to study in more detail later. Little-known modern graphical methods can be powerful tools for exploration of complex data. We illustrate several such types of graphs and discuss what we have learned from them. The data come from a study designed to quantify physical and enzymatic changes in both sides of chicken brains after developmental exposure to dioxin, with half the eggs treated with red light. The data corresponded to 160 combinations of dose, light presence or absence, brain side, and brain dimension or biochemical activity, making the graphical methods especially helpful for assessing the presence and extent of asymmetry in the responses. We provide graphical examples using a pairs plot and several trellis plots in which the data appeared consistent or inconsistent with prior expectations and suggested unexpected findings that will motivate new research.