Review of cellular and subcellular changes in the cambium
The commonest approach to studying cambial productivity is conventional light microscopy, which is widely used in wood formation studies. The number of such studies has increased rapidly in the past decade, usually in order to elucidate the relationship between growth and environmental factors. However, some aspects of cambial seasonality are often overlooked or neglected. Observations with transmission electron microscopy provide a more detailed insight into changes occurring on the ultra-structural level in cambial cells. Criteria for defining cambial activity are not yet fully clarified, especially when observing it at different resolutions, i.e., on cellular, subcellular and ultrastructural levels. The goal of this review is to contribute to clarification of the terms mainly used, such as cambial dormancy, reactivation, activity, productivity and transition between different states, resting period and quiescence, which describe structural modifications of cambial cells during the various phases of their seasonal cycle. Based on our own cambium observations on adult beech trees growing at two different elevations, which were made with light and transmission electron microscopy, we discuss the influence of weather conditions on cambial activity and the advantage of the complementary use of different techniques and resolutions.