Basics of Evolution - Korzh O.P. - 2006

Part III. PATHS OF LIFE DEVELOPMENT

Chapter 19. Main Trends in the Development of Living Organisms

19.2. Transition of Plants to Land

Terrestrial life conditions differ significantly from those in water: on land, higher plants simultaneously exist in two fundamentally different environments—their aerial parts in the atmosphere, and their underground parts in the soil.

The aerial environment features a significantly higher oxygen content than water, while the conditions of mineral Nutrition and water regime (constant water exchange between plants and the environment) in the soil are entirely different. Therefore, the transition of higher plant ancestors into these new living conditions could only occur after they acquired specialized adaptations.

One of the first, and perhaps the main adaptation of higher plants to terrestrial conditions, was the appearance of the epidermis—a special protective layer consisting of tightly packed Cells of the dermal tissue (Fig. 19.2). It prevents plant desiccation, the NEGATIVE IMPACT OF Temperature fluctuations, the penetration of various pathogens, and so on. The epidermis arose through the modification and specialization of the outer cells of the plant body. Their surface in most higher plants is covered with a specialized protective film—the cuticle—which is chemically resistant and possesses water-repellent properties. If the cuticle layer were continuous, it would deprive plants of The ability to gas exchange with the environment, leading to their death. Consequently, small specialized structures—Stomata—evolved, through which intensive Diffusion of Water vapor, oxygen, and carbon dioxide takes place. The presence of stomata is one of the fundamental features of higher plants, starting from rhyniophytes. Both stomata and epidermis are almost absent in the Dermal Tissues of roots and in secondary aquatic plants, which should be regarded as the result of the reduction of the latter.

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Fig. 19.2. Stem of Psilotum nudum (from P. Raven, R. Evert, and S. Eichhorn, 1990):

A - Cytology/practical/72.html">Cross section of the stem; B — protostele

The Emergence of dermal tissue was not the only prerequisite for the transition of plants to terrestrial life. The Organization OF THE epidermis meets two opposing requirements: on the one hand, preventing the desiccation of the plant body, and on the other hand, allowing air with carbon dioxide to reach green cells containing METABOLISM/14.html">Chloroplasts. At the same time, stomata not only let in carbon dioxide but also allow water to evaporate freely during Transpiration. Therefore, at the very beginning of higher plant evolution, the need arose to maintain their water balance at an appropriate level.

Higher plants became relatively independent of moisture fluctuations in the soil and atmosphere after acquiring the ability to stabilize the water content within their bodies. Solving this problem led to The formation of xylem, which is specially adapted to perform water-conducting Functions. Phloem elements also appear, but analogous structures already existed in Algae, which is why phloem is considered evolutionarily much older than xylem.

As an Organism increases in size, the surface-to-volume ratio changes; to maintain this ratio at an optimal biological level, the plant body becomes segmented. W. Zimmermann proposed the telome theory of THE ORIGIN OF modern higher plant Organs, which is currently supported by almost all botanists. According to this theory, the organs of most plants formed from a collection of rhyniophyte telomes through flattening, overtopping, webbing, and reduction of telomes (Fig. 19.3).

Fig. 19.3. Rhynia (Rhynia gwynne-vaughanii / Rhynia major) (1) and diagram of the origin of a fern sporophyte from a Rhynia-type telome (2-5) (I.M. Hryhora et al., 2004)

These processes caused the Differentiation of the sporophyte (the asexual diploid generation) and the division of functions among its parts, as well as the formation of a specialized organ containing spores—the sporangium, which developed through the modification of branch tips. Its wall becomes sterile and protects the internal contents from desiccation.

On land, reproduction via spores acquires special significance, since the vast majority of them fall into conditions unsuitable for germination.

Unlike the aquatic environment, the spore wall becomes rigid, poorly permeable to water, and performs a protective function. As a consequence, under terrestrial conditions, plants face the need to produce A large number of spores, which requires sufficient accumulation of organic matter and a corresponding increase in sporophyte size.

Simultaneously with the Formation of the protective sporangium wall, Structure/149.html">The problem of spore release and their subsequent dispersal in the environment arises. In rhyniophytes, spore dispersal initially occurred after the sporangium walls ruptured, but later specialized mechanisms emerged that gradually solved the problem of asexual reproduction (Fig. 19.4).

Fig. 19.4. Sporangia and spores (from I.V. Honcharenko, 2004):

A - rhyniophytes; B - heterosporous lycophytes; C - horsetails (sphenophytes); D - homosporous ferns; 1 - terminal sporangium; 2 - sporangium wall; 3 - sporogenous tissue; 4 - microsporangium; 5 - megasporangium; 6 - microspore; 7 - megaspore; 8 - sporangia on sporangiophores; 9 - spore with elaters; 10 - marginal sporangia; 11 - sporangium

Evolutionary Processes affected the internal structures of plants: tissues present in rhyniophytes underwent further development, and new ones were formed. Conducting systems were improved, with vessels being considered the most advanced element (vascular plants). Due to the need to maintain an upright body position, plants developed stem sclerenchymatous fibers, wood and bast fibers, etc. Gradually, the cambium appeared, ensuring Secondary Growth and the thickening of stems, branches, and roots.

In aquatic algae, there is no problem with the Fertilization of female Gametes (egg cells) because male gametes are capable of moving freely. With the transition of plants to land, the need arises to protect reproductive organs from desiccation and to ensure the Sexual process in new conditions. Under these circumstances, the gametangium undergoes certain modifications: the outer layer of the wall becomes sterilized and, just like in the sporangium, performs a protective function (leading to the formation of antheridia—male, and archegonia—FEMALE REPRODUCTIVE ORGANS; Fig. 19.5).

Fig. 19.5. Structure of antheridia and archegonia, and the fertilization process in a bryophyte moss (after P. Raven, R. Evert, and S. Eichhorn, 1990)

Yet, the formation of an external protective layer in gametangia does not resolve all the challenges of sexual reproduction, as sperm motility remains entirely dependent on water. Consequently, the combination of a sessile lifestyle with the fertilization of sex cells posed a major obstacle that significantly hindered the spread of vascular plants onto land. As a result, this led to a diminished role for the gametophyte and its gradual reduction in the vast majority of higher plants.

It is generally believed that all higher plants originated from ancestral algae (Fig. 19.6). Evidence for this includes the similarities between rhyniophytes—the most ancient and long-extinct representatives of higher plants—and algae, particularly in their branching patterns. Other shared features encompass the Structure and function of chloroplasts, the Morphology/12.html">ALTERNATION OF GENERATIONS in Higher Plants and multicellular algae, and the presence of flagella and self-motility in the male gametes of many higher plants.

Fig. 19.6. Diagram of evolutionary relationships among higher plant divisions (after I.V. Grushvitsky et al., 1978)

It is hypothesized that higher plants likely originated from freshwater or brackish-water green algae. Evidence supporting this hypothesis includes the following: chlorophyll $a$ is the primary photosynthetic pigment in both groups, while chlorophyll $b$ and carotenoids serve as accessory pigments; Cellulose is a key component of The Cell wall; and starch is stored within chloroplasts rather than in the Cytoplasm, as seen in other algae. Algae exhibit A wide variety of ratios between the diploid and haploid phases of The life cycle. However, in the algal ancestor of higher plants, the diploid phase was likely more developed than the haploid phase.

The most ancient and primitive division of higher plants is the Rhyniophyta, which emerged in the Silurian and persisted into the Late Devonian. Due to their extreme simplicity of organization, some rhyniophytes morphologically resembled algae more than higher plants. They were characterized by terminal sporangia at the SHOOT apices, the absence of a true ROOT system and leaves, dichotomous or pseudomonopodial sporophyte branching, a primitive Anatomical Structure, and homospory (The production of spores identical in size and physiological characteristics).

Their Vascular System was weakly developed, consisting of a continuous strand of conducting tissue with a central xylem core entirely surrounded by phloem—representing the most primitive type of stele (central cylinder). The xylem consisted of tracheids; unlike most vascular plants, rhyniophytes lacked mechanical (supporting) tissues. Even at this early stage, these plants featured a small number of simply structured stomata, each comprising only two guard cells and a stomatal pore. Since the meristem was restricted to the shoot apices, rhyniophytes lacked secondary growth capabilities.

The sporangia of rhyniophytes were thick-walled and varied in shape and size. They produced numerous spores that were structurally typical of higher plants, though specialized mechanisms for sporangial dehiscence were either absent or very primitive. In *Rhynia*, the sporangia were still virtually indistinguishable externally from the tips of ordinary shoots.

The absence of gametophyte fossils for these plants leaves our understanding of their organization somewhat incomplete. Based on comparative anatomical data from the gametophytes of modern plants, scientists suggest that the rhyniophyte gametophyte was well-developed, predominantly subterranean, mycorrhizal (living in Symbiosis with Fungi), dichotomously branched, and possessed a vascular system; however, the exact details of its structure remain an open question.