Fundamentals of Evolution - O.P. Korzh - 2006

Part III. PATHS OF LIFE DEVELOPMENT

Chapter 19. Main Trends in the Development of Living Organisms

19.5. Flowering Plants as the Pinnacle of Higher Plant Evolution

Flowering plants (angiosperms) constitute the most species-rich division of the plant kingdom, encompassing approximately 250,000 species. They inhabit every climatic zone and a vast array of ecological niches, ranging from tropical rainforests to tundras, from wetlands to deserts, and from coastal areas to high-altitude alpine regions. These plants have successfully reached the apex across all directions of adaptation to terrestrial environments.

First and foremost, this is reflected in the profound specialization of their Tissues and Organs. The Vascular System, particularly the xylem, develops true vessels that significantly enhance Water transport efficiency compared to tracheids, along with specialized thick-walled tracheids that provide mechanical support. The phloem also undergoes major evolutionary transformations, with companion Cells emerging alongside sieve tubes. Storage tissues begin to aggregate around conducting elements, thereby increasing their functional efficiency. Initially, the single central cylinder (stele) breaks down into multiple discrete components (vascular bundles), which facilitates the differentiation of plant organs.

A clear division of labor among organs enables plants to adapt much more effectively to specific environmental conditions, as identical Functions can be performed by different structures. For instance, leaves, stems, and roots can all participate in nutrient storage, asexual reproduction, and anchoring the plant relative to its environment, while both stems and leaves also take on photosynthetic duties (giving rise to ANALOGOUS AND HOMOLOGOUS organs).

Extensive branching of the stem, intricate leaf structures and shapes, and the specialized architecture of the SHOOT maximize the utilization of vital resources not only for the individual plant but for the entire plant cover. It is precisely in these plants that vertical stratification (tiering) begins to play a crucial role; they proved to be the only plant group capable of forming complex multi-layered communities. This promotes a more thorough exploitation of habitats and a rapid colonization of new territories.

The most profound transformations occur within the generative sphere of angiosperms. Above all, this involves The formation of a fundamentally novel and highly complex system—the flower.

Its functions encompass micro- and megasporogenesis, pollination, Fertilization, embryo development, and fruit formation. All seed plants possess an ovule, which is fundamentally a modified megasporangium. However, its Structure, the Specifics of the sexual process, and seed development vary considerably between gymnosperms and angiosperms. In the latter, gametophytes undergo maximal reduction and ultimately lose their sex organs. Thus, this aromorphic feature arises as a consequence of the near-complete degeneration of the sexual generation in flowering plants. Specialized floral adaptations significantly increase the probability of successful fertilization, while the formation of a protective seed coat (the pericarp) greatly enhances seed viability. The fruit fulfills two distinct functions: protecting the ovule and disseminating the seeds.

The difficulties associated with tracing the evolutionary origin of flowering plants stem from the lack of intermediate links bridging them to other groups of higher plants. Consequently, understanding the Origin of the Generative organs of this plant Lineage—foremost the flower—is of paramount importance. While this problem has not yet been definitively resolved, two primary theories regarding floral morphogenesis are most widely accepted: the pseudanthial (false flower) theory and the euanthial (true flower) theory.

The pseudanthial (false flower) theory was proposed by R. Wettstein. According to this hypothesis, the flower originated as a simplified inflorescence derived from a cluster of unisexual male and female "flowers" of advanced gymnosperms, which subsequently evolved through progressive refinement from a single-integumented anemophilous type to an entomophilous type featuring a double perianth and a fixed, stable number of constituent parts.

The euanthial (or strobiloid) theory posits a direct historical connection between the flower and the strobili of extinct gymnosperm groups (likely bennettitaleans). According to this view, the archaic bisexual strobilus of bennettites served as the ancestral template for The Development of the reproductive organs in both gymnosperms and angiosperms. It differed significantly from the angiosperm flower, primarily in The structure of its micro- and megasporophils. Nevertheless, it can arguably be viewed as a structural prototype for the flowers of modern magnolias, water lilies, and other relatively primitive flowering plant lineages.

Various systematic groups of gymnosperms and even HIGHER SPORE PLANTS have been proposed as potential ancestors of angiosperms. However, comparative analysis of bennettitalean strobilus structure and floral Morphology rules out direct Homology between them. Furthermore, hypotheses deriving flowering plants from conifers, gnetophytes, caytoniales, cordaites, cycads, and other groups have been dismissed. It is possible that angiosperms descended from an as-yet-undiscovered ancestral gymnosperm lineage. The Theory of hybridogenic origin of flowering plants is also of certain interest.

The telome theory suggests the parallel Formation of the flower alongside the vegetative shoot from the telomes of psilophytes (Fig. 19.3). However, this concept largely falls within the framework of the euanthial theory.

The MAIN STAGES OF floral evolution are envisioned as follows:

1. The inward folding of megasporophils (seed scales) and the fusion of their margins. Thus, from megasporophils where ovules lie exposed (in gymnosperms), a pistil emerges in which the ovule is enclosed within the Ovary (angiosperms). Fruits develop from megasporophils (referred to as carpels in angiosperms).

2. Microsporophils also undergo metamorphosis: they become leaf-like, the number of microsporangia is reduced to four, and they fuse in pairs (forming a four-sporangiate anther). The leaf-like microsporophil transforms into the stamen filament.

3. Gamete formation and the Sexual process in angiosperms undergo drastic alterations, with fundamental shifts affecting female gametophyte development, pollination, and fertilization.

The remarkable diversification in both the external and internal Organization of flowering plants suggests the independent origin of their various groups from multiple gymnosperm ancestors, and occasionally even from different divisions of higher plants. However, the hypothesis of a polyphyletic origin contradicts modern scientific understanding. The unified origin of all plants within this division is evidenced by the presence of a substantial number of shared general morphological, anatomical, and embryological traits among representatives of different angiosperm families and orders. On this basis, it is concluded that flowering plants are monophyletic in origin.

The direct ancestors of these plants also remain unknown. None of the extant plants possess the full suite of primitive traits characteristic of primordial flowering plants, as even relatively primitive representatives of various taxa have adapted to specialized conditions. From the perspective of comparative morphology, primordial flowering plants were woody taxa, most likely small trees or shrubs. Their leaves were evergreen, arranged alternately on the shoot, exhibited xeromorphic features, and possessed simple pinnate venation. Their bisexual flowers were aggregated into primitive terminal inflorescences; sepals, stamens, and pistils were arranged spirally, and the fruit was an aggregate of follicles.

In light of the above, most researchers believe that the earliest angiosperms resembled modern polycarpic plants, most likely approaching the Magnoliales. However, from the very onset, the EVOLUTION OF FLOWERING plants proceeded through broad adaptation (adaptive radiation) at a rapid pace, enabling them—already by the mid-Cretaceous—to achieve immense morphological diversity and adapt to the most varied environmental conditions. Insect pollinators (coevolution) and seed-dispersing birds played a particularly pivotal role in the dissemination of flowering plants, with fruit-eating animals becoming crucial at later stages.

An Analysis of the geographical distribution and phylogenetic affinities of the most archaic living flowering plants indicates that the likely primary center of formation and diversification for these plants is Southeast Asia—one of the most tectonically complex regions on Earth. It is possible that certain parts of Laurasia were initially connected to East Gondwana and drifted northward during the Jurassic period. It cannot be ruled out that flowering plants as a group arose precisely on one of such initially isolated landmasses.

The angiosperm division is traditionally subdivided into two classes: dicotyledons and monocotyledons. The chronological sequence of The Emergence of these classes and lower taxonomic ranks remains a subject of ongoing scientific debate. The general evolutionary trajectory of these plants is characterized by a transition from woody to herbaceous life forms, a shortening of The life cycle, a reduction and stabilization of floral organ numbers (driven by stabilizing natural Selection), and other structural reorganizations. However, different taxonomic lineages resolved these evolutionary challenges in their own distinctive ways.