Paleobotanists studying fossil plants of distant geological eras deal with scattered parts of a plant: leaves, inflorescences and seedlings, fragments of wood. From them, the appearance of ancient flora and vegetation is restored and an idea of the climates of the past is formed.
Fossil plants, depending on the conditions of fossilization (fossilization), can be represented by imprints of organs on the rock, bearing only to a varying degree informative information about the general morphology of the remnant, may have phytoleims – carbonaceous films, which, after certain exposure to chemicals, supplement the knowledge of anatomy; also, remnants can be mummified, and then it is possible to obtain more complete information about the volumetric structure of the object. The remains of leaves of ancient plants are found in the fossil record more often than other phytofossils.
Knowledge of the morphology and anatomy of modern plants often helps to determine which group of plants a particular fossil belongs to. In the case of leaves, paleobotanists have to take into account the possible variability of their characters, which is characteristic of many taxa. This knowledge helps to understand whether the leaves belong to the same or different types of plants and to draw the correct conclusion about the diversity of the fossil flora. One of the most important features of assessing the taxonomic weight of a trait is to determine the degree of its variability, an idea of the amplitude of its variation at different stages of leaf development and under different parameters of the climate in which the plant exists.
Microclimatic conditions in different parts of the tree crown (central and peripheral parts) differ in the degree of illumination, heating, exposure to air currents. In addition, the leaves of different height tiers of the crown are in unequal water supply conditions due to the fact that the flow of water into the upper tiers is difficult due to the need for the plant to overcome the force of gravity. The existing horizontal and vertical gradients of environmental factors determine the diversity of macromorphological and microstructural traits of leaves in the crown of a woody plant.
Investigations of the complex of signs of morphology and anatomy of leaves obtained from different zones of a well-developed crown of a sycamore tree, which is characterized by both lobed and whole leaves, made it possible to draw up a “map” of the crown. As it turned out, lobed leaves with differently developed lobes, a solid or toothed edge are confined mainly to the periphery and center of the crown, while asymmetric leaves with a lobe formed only on one side are located mainly inside the crown, and whole leaves prefer extremely deep areas … In addition, the leaves inside the crown, experiencing a deficit of light, have more elongated leaf blades with relatively smaller teeth of the edge, and, conversely, wider plates with larger teeth are found in leaves located at the periphery of the crown, in its more illuminated areas. Significant differences between the “light” and “shadow” leaves were found in the study of their anatomical and physiological parameters. In particular, both quantitative (number of stomata and hairs per unit area, cell size) and qualitative (thickness and features of the cuticle, shape of cell walls) differences are observed in the structure of their integumentary tissues.
Many of these features of “light” and “shadow” leaves were previously known, but it was not clear how general they might be. In this regard, similar studies of tree crowns from other taxonomic groups, in particular, liquidambar, as well as evergreen oaks, with different leaf morphology, were carried out. Unlike plane trees, the leaves from these trees were not so diverse in shape and other external features, and it was all the more interesting to understand whether there was any regularity in their distribution in the crowns. The results of the study of signs of morphology and anatomy, reliably confirmed by statistical methods, showed that the morphotypes of “light” and “shadow” leaves can be confidently distinguished, and the “map” of the tree reflects well their distribution in the developed crown. In other words, leaves from one tree can differ to varying degrees in morphology and anatomy due to the fact that a woody plant with a voluminous crown is forced to adapt to the heterogeneous microclimatic conditions in which its various organs find themselves.
As the “map” of the modern sycamore clearly shows us, leaves from one tree can show multiple variations in morphological traits. Taken separately, they may well be attributed to different types of plants! Imagine now that in one locality of fossil flora a spectrum of leaves similar to that depicted on the “map” was found. The question immediately arises: do these leaves belong to the same plant or to different ones? Relying only on visible morphological characters, it is quite possible to assume that very different plants, belonging not only to different genera, but even to families and orders, grew in this locality. The key to solving this puzzle is given by the “map” of the modern tree, which serves as a model for paleobotanists. Comparison of the complexes of leaf traits established for “light” and “shadow” leaves in the studied fossil leaves, as well as the presence of transitional forms between them, allows us to combine all the findings into one species. With this approach, the paleobotanist can more accurately identify fossil leaves and, on this basis, gain an idea of the true diversity of the flora of the past.
The well-known aphorism “the amazing is near” has become so firmly established in life and became commonplace that, behind the daily bustle, a person ceased to feel it as a real prompt to see and learn something new. A pedestrian who is not often in a hurry on urgent matters looks around, examines the nature around him. But sometimes a casual look can become a reason for future full-fledged research, the results of which will be useful to science.
So it was when, on the way home from work, the paleobotanist saw the crown of a large tree with carved leaves, clearly looming against the background of the sky in the rays of sunset. At first glance, the leaves that were identical, as it seemed, still had some differences. Testing this guess through a scientific experiment showed that the crowns of trees are not at all randomly organized, as it might seem at first glance. The dictum belongs to the ancient Roman politician and philosopher Cicero: “There is nothing more inventive than nature.” Everything in nature has meaning, and our task is to understand this great design.