Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
3.9. Leaf Formations, Heterophylly, Leaf Mosaic. Leaf Fall

Leaf formations. In the course of phylogeny, plants have developed various categories of leaves, which are classified into lower, middle, and upper formations. The lower formation includes cotyledons, rhizome scales, and reduced leaves of tubers and bulbs; all of these perform protective or storage Functions. The middle formation comprises photosynthetic leaves, which are the most characteristic of the plant and constitute its bulk. The upper formation consists of underdeveloped leaves of the upper SHOOT regions, bracts, bracteoles, and stipules, whose primary role is to protect the reproductive Organs.

Changes in the shape and Structure of leaves at different shoot nodes from bottom to top are referred to as heterophily (as seen in Water crowfoot and arrowhead). When variations in leaf size and shape (less commonly) occur at a single node, but the leaves are oriented differently relative to the horizon and light, this phenomenon is known as anisophily. Heterophily can be ecological, when different blade shapes are determined by environmental factors (such as the submersed and emersed leaves of buttercups or Salvinia), or phylogenetic, when driven by the historical Development of Plants. The presence of lower leaves differing in shape from the middle ones in linden, hornbeam, and maple indicates that their ancestors possessed leaves similar to these lower ones.

Leaf mosaic. Leaf mosaic refers to the arrangement of leaves of varying sizes in such a way that they do not overlap one another, thereby creating optimal conditions for illumination. Sometimes upper leaves feature deep lobes or incisions that allow light to penetrate to the leaves situated below (as in Monstera).

In the course of evolution, as an ADAPTATION TO ENVIRONMENTAL conditions, the leaves of certain plants underwent modifications (metamorphoses). The primary leaf metamorphoses include: spines (barberry, cactus), which function either protectively or as an adaptation to reduce water evaporation; tendrils (pea, vetch), whose main function is attachment to support; and phyllodes, where the petiole becomes flattened and functionally replaces the blade. The leaves of carnivorous plants have undergone fascinating transformations: for example, in the Venus flytrap, the blades of basal leaves have evolved into trapping organs, whereas in bladderwort, a part of the leaf is modified into a bladder, and so on. Metamorphosed leaves also include the sepals, petals, stamens, and pistils of flowers, as well as the fleshy leaves of cabbage and bulb scales.