Large seasonal variation in the rate of change in girth of tropical rain-forest tree species has been described, but its origin is still under debate. We tested whether this variation might be related to variation in atmospheric relative humidity through its influence on bark water content and thickness. Variation in trunk circumference of 182 adult trees was measured about twice a month in an undisturbed tropical rain forest over 18 mo using dendrometers. Furthermore, a laboratory experiment was conducted to test the direct influence of relative air humidity on bark water content and thickness. In the field, most trees displayed highly positive rates of change in girth at the onset of the wet season, while a quarter of the trees displayed negative changes during long dry seasons, whatever their total annual growth. This variation was correlated with environmental conditions, particularly with atmospheric relative humidity. Trees with high bark water content and thickness displayed a stronger decrease in girth during the dry season. In the chamber experiment, desiccation induced a decrease in the diameter of the trunk sections in tandem with a decrease in bark water content. As a result, seasonal variation in the rate of change in girth of tropical rain-forest trees reflects variation in trunk biophysical properties, through the influence of relative humidity on bark properties, but not directly variation in secondary growth.

Plants express evolutionary adaptations in response to overall environmental stresses (selection pressures), of which insect infestation usually represents only a minor component. The majority of insect-host plant relationships involves adaptations of insect species to existing plants. Over time, plants have evolved a variety of physical and chemical defence mechanisms against environmental stresses. Only some of these are active against insects, and then not against all species. Plants rarely seem to evolve defences specifically against insects. Parameters of plant resistance to parasites are classified as constitutive or inducible. Soybean resistance to agromyzid stem-miners involves both constitutive and inducible components. Expression of such resistance parameters involves controls ranging from single genes (e.g. for trichome traits) to various combinations of genes (e.g. for the metabolic pathway for phenyl-propanoids). Environmental stresses (e.g. high illumination) especially affect the expression of resistance parameters which involve dynamic metabolic pathways. Soybean (Glycine soja) resistance to beanflies is an example of ‘horizontal’ resistance. Thus, the expected stability is long-term. Such stability is attributable to its major parameters being fundamental components of the soybean's primary and secondary growth and differentiation. These kinds of parameters are major products of the soybean's successful evolutionary experience at survival.