PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 4. THE EVOLUTION OF PLANT BODY FORM

4.1. The Concept of Evolution. The Evolution of Body Form in Primitive Organisms

Even upon the most superficial acquaintance, the plant world strikes one with its staggering diversity. From microscopic unicellular Algae to the giants of the plant kingdom—such as sequoias reaching 100 meters and more in height; from aquatic plants to desert succulents; from the tiny flowering plant *Wolffia*, measuring no more than 1 mm with a flower of about 0.5 mm, to *Rafflesia tuan-mudae*, whose flowers reach up to 1 meter in diameter, and so on. Such comparisons could go on indefinitely, as scientists have currently cataloged approximately 40,000 species of algae and 300,000 species of higher plants, with new species still continually being discovered.

How could such diversity have arisen? This is one of the fundamental problematic questions that plant Morphology must answer. Science responds unequivocally: the entire Water/126.html">Diversity of the plant world could only have emerged through The process of evolution.

What should be understood by evolution? Walter Zimmermann (1953) provided the following Definition of the concept: "The transformation of the form and mode of life of organisms, As a result of which descendants acquire new traits that distinguish them from their ancestors." This definition essentially describes the final outcome of the evolutionary process rather than revealing its underlying causes. A more detailed definition belongs to the Soviet zoologist S. S. Shvarts (1974): "Evolution is the historical development of living beings accompanied by the progressive perfection of their morphophysiological Organization. Ultimately, evolution is a process of adaptation to the environment." This latter statement emphasizes and clarifies the driving force behind the transformations that occurred both in aquatic inhabitants and in plants that made the transition to land.

According to prevailing views, eukaryotic organisms appeared approximately 1 to 1.5 billion years ago in an aquatic environment. It is hypothesized that they were spherical and fed heterotrophically, absorbing nutrients across their entire body surface. Among contemporary unicellular algae, one can

encounter quite a few representatives with a spherical body form (*Chlorella*, *Eremosphaera*, *Golenkinia*, etc.). Gradually, under the Influence of Environmental conditions, the body form of algae underwent changes. Since food absorption from the external environment was carried out by the entire body surface, those organisms with a larger surface area were at a distinct advantage.

Thus, the mode of Nutrition should be regarded as the primary driving factor of evolution, which dictated the Changes in the body form of ancient algae. The core principle of plant body form evolution consisted in the maximization of their surface area. Given that the overwhelming majority of plants are autotrophic organisms, evolution at its final stage ensured the maximization of the plant's photosynthetic surface.

In the Cytology/cytology/16.html">Early stages of the evolution of spherical organisms, an increase in surface area evidently occurred while geometric similarity of the body was preserved—that is, the organisms remained spherical. However, spherical organisms could not grow indefinitely. While maintaining geometric similarity, the volume and surface area of spherical bodies grow disproportionately. The surface area increases in an arithmetic progression, whereas the body volume increases in a geometric progression. This leads to a disruption of metabolic exchanges between the unicellular Organism and its environment, creating unfavorable conditions for survival that can ultimately lead to death. To avoid destruction and continue developing successfully, the spherical plant body had to undergo some form of modification. Based on an Analysis of the body Structure of modern algae, it can be hypothesized that during adaptive evolution, body volume and surface area could be reconciled via two distinct pathways. In some organisms, the increase in body volume led to a transition from a spherical shape to alternative geometries. In others, the enlargement of the surface area was coupled with intensive nuclear division unaccompanied by subsequent cytokinesis. This gave rise to unicellular multinucleate organisms that differ substantially in appearance, structure, and size from typical Cells and are referred to as siphonous algae (*Botrydium*, *Caulerpa*). However, evolution did not follow the path of increasing complexity and further development of siphonous forms; this evolutionary branch proved to be a "dead end." In contrast to the second direction, the first pathway proved to be highly progressive and gradually led to The Emergence of forms possessing a larger surface area per unit volume than a sphere. This is how cylindrical, rod-shaped, sickle-shaped, elliptical, and other unicellular organisms originated, which persist among algae to this day.

Multicellular Organisms presumably evolved from unicellular ones. The Role of an intermediate link between them was played by colonial forms—aquatic organisms in which the offspring of asexual reproduction remain attached to the maternal organism, forming a more or менее complex association. Colonies can be either free-floating or attached. Free-floating colonies have a spherical shape. Attached colonial algae may be filamentous, sac-like, sac-like-lamellar, or arborescent. In primitive colonies,

cells are evenly distributed within the thickness of the enveloping mucus and perform identical Functions. The advantage of colonial forms over unicellular organisms lies in the fact that cells within a colony can maximally utilize nutrients or metabolic byproducts of other cells, primarily oxygen and carbon dioxide.

As environmental conditions changed, The structure of colonial forms became more complex: evolution not only progressed via alterations in the colony's external appearance, but was also linked to the functional differentiation of its constituent cells. Alongside free-floating colonies, attached ones appeared. In such colonies, the capacity for Cell Division is retained solely by certain apical cells, while the basal cell performs the function of attachment to the substrate. If the apical cell divides in only one direction, the colony acquires a filamentous form (*Ulothrix*). If division proceeds in two dimensions (length and width), single-layered or double-layered lamellar forms arise (*Ulva*, *Enteromorpha*). When division occurs in three dimensions, multi-layered lamellar organisms are formed (*Fucus*, *Laminaria*).

Despite their complex external structure, multicellular algae possess a rather primitive internal organization. They lack true Tissues. The plant body branches dichotomously (from the Greek *dicha* – in two parts, *tome* – cut, section), and it is not differentiated into vegetative Organs. Reproductive organs (sporangia, gametangia) in the overwhelming majority of algae are unicellular (multichambered gametangia are extremely rare). A plant body that is not differentiated into vegetative organs is called a thallus.

Thalloid organisms are also found among primitive higher plants (certain liverworts), but they are few in number and their evolutionary significance is relatively minor (Fig. 52).

Class="center">Fig. 52. Liverwort *Marchantia polymorpha*: 1—thallus with female archegoniophores; 2—thallus with male antheridiophores

4.2. Reconstructed Diagram of the Structure of Psilophytes (Rhyniophytes)

The culmination of the evolutionary process was the establishment on Earth of higher plants characterized by the SHOOT-and-leaf organization inherent to the majority of them. The emergence of higher plants was the result of ancestral algae moving onto land and adapting to existence in an aerial environment.

The earliest reliably known representatives of land plants are rhyniophytes (psilophytes). The organization of rhyniophytes is extremely primitive. Many of them (*Cooksonia*, *Rhynia*) closely resembled algae, had small dimensions (from 20 to 100 cm), branched dichotomously, and lacked any differentiation into leaves and stems (Fig. 53). At the same time, these plants exhibited clear features distinguishing them from algae. These land pioneers developed certain tissues: a dermal tissue with well-defined Stomata, parenchymatous tissue, and, most importantly, Vascular Tissues—xylem and phloem.

Fig. 53. Land pioneers: A—Cooksonia; B—psilophyte; C—Rhynia; D—Asteroxylon

The first clear concepts regarding the structure of land pioneers were provided by the founder of telome theory, W. Zimmermann, in his book *Phylogeny of Plants* (1930). Since extinct plants are most frequently discovered in the form of fragmented remains, he proposed a reconstructed diagram of psilophyte structure. According to W. Zimmermann's views, the body of psilophytes was not yet differentiated into organs and consisted of axial, dichotomously branched aboveground and subterranean structural elements. Plants attached themselves to the substrate via dichotomously branched, soil-submerged rhizomoids (from the Greek *rhiza* – ROOT and *eidos* – form). Rhizoids formed on the rhizomoids, through which water with dissolved nutrients was absorbed from the soil into the plant. The aboveground portion of the plant consisted of mesomes (from the Greek *mesos* – middle) and telomes (from the Greek *telos* – end), which were terminal "branchlets" bearing sporangia at their tips. Mesomes performed the functions of Photosynthesis and nutrient conduction, and were situated between the telomes and rhizomoids (Fig. 54).

The transition of plants to land was accompanied by The Development of a complex system of adaptations to new living conditions, which fundamentally altered not only their outward appearance, but also their internal structure.

Fig. 54. Diagram of psilophyte structure: 1—sporangia; 2—telomes; 3—mesomes; 4—rhizomoids; 5—rhizoids

In the course of evolution, the Structural elements of the first land pioneers gave rise to the Vegetative organs of higher plants. Rhizomoids evolved into true roots, mesomes transformed into stems and lateral branches, and telomes either retained their function as spore-bearing structures or gave rise to leaves. Thus, through prolonged evolution, cormophytes (leafy plants) emerged, highly adapted to terrestrial life. The driving forces behind the morphological changes in higher plants undoubtedly included nutrition requirements (both soil and aerial) and environmental conditions. The fundamental principle governing the Evolution of the plant body was the enlargement of the photosynthetic surface.