PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 4. THE DEVELOPMENT OF PLANT BODY FORM

4.3. Changes in the Morphological Structure of Plants. Evolution of Branching Types

Changes in the morphological features of higher plants are primarily associated with the modification and refinement of branching patterns, as well as The Emergence of specialized photosynthetic Organs—leaves.

Paleobotanical studies have established that dichotomous branching was the initial type. This Conclusion was first reached in 1912 by the German botanist H. Potonié (1857–1913). In dichotomous branching, lateral branches form through the division of the apical meristem, which is why it is also referred to as apical branching. With dichotomous branching, the number of lateral branches increases in a geometric progression. A first-order axis produces two second-order axes, two second-order axes produce four third-order axes, and so on. As a result, the plant develops multiple apexes, causing it to spread horizontally. Plants with dichotomous branching never grow tall.

There are two types of dichotomous branching: isotomous (from the Greek isos meaning equal) and anisotropous (from the Greek anisos meaning unequal). In isotomous dichotomy, both branchlets formed by the division of the apex of each axis grow evenly and reach the same length. This type of branching is typical of plants inhabiting relatively stable environments (such as Water or dense stands of vegetation). Among extant higher plants, isotomous dichotomy is found in floating liverworts (Riccia fluitans) and Representatives of the genera Diphasiastrum and Huperzia from the phylum Lycopodiophyta. Under the Influence of Environmental conditions, higher plants developed anisotropous dichotomy, in which the resulting branchlets began to develop unevenly, causing one to outgrow the other. Anisotropous dichotomous branching has been discovered in certain fossil plants as well. It can be viewed as the onset of The process of overtopping—the transition of a plant from multiple apexes to a single apex, and consequently, to the establishment of a single main axis. Among living plants, anisotropous dichotomy is characteristic of staghorn clubmoss (Lycopodium clavatum) and several other lycophytes. A marked lag in the growth of one of the branchlets led to the appearance of dichotompodial branching (from the Greek dicha meaning separately and pus (podos) meaning FOOT, referring here to the axis), which produces a main zigzag, vertically oriented axis. Dichotompodial branching was also characteristic of the fossil plant Asteroxylon. The emergence of this branching type must be regarded as a major milestone in the evolution of land plants. In plants with dichotompodial stems, the sporangia were elevated significantly above the ground, ensuring the efficient dispersal of spores (Fig. 55).

Class="center">Fig. 55. Diagram of apical branching types: A—isotomous dichotomous; B—anisotropous dichotomous; C—dichotompodial (numbers indicate sequentially developing shoots)

The process of overtopping that began in ancient plants ultimately consolidated The formation of the main axis and led to the evolution of monopodial branching (from the Greek monos meaning single, sole). In monopodial branching, the main axis can grow in length relatively indefinitely for quite some time, which contributed to an increase in plant height. At the same time, the pattern of lateral branch formation changed: instead of originating from the apical meristem, lateral branches began to arise from lateral (and more rarely adventitious) buds. For this reason, this type of branching is also called lateral branching. Like the main axis, the lateral branches exhibit relatively indeterminate growth. Due to these growth characteristics, an unlimited number of leafy lateral branches are formed on axes of various orders, which dramatically increases the photosynthetic surface area. Because The Development of lateral branches in monopodial plants most commonly occurs in an acropetal direction (from the base to the apex), plants—especially trees—tend to develop more robust lower lateral branches, forming a pyramidal crown. This type of monopodial branching is termed basitonic. In certain acacias, monopodial branching results in more vigorous Development of the upper branches, giving the crown an umbrella-like shape; this is known as acratonic monopodial branching. In mesotonic branching, the strongest and longest branches are produced in the middle region of the main SHOOT. This monopodial branching pattern gives plants a columnar crown shape. Monopodial branching is a fairly ancient type, characteristic of the vast majority of gymnosperms (Fig. 56).

Fig. 56. Diagram of various types of monopodial branching: A—acratonic branching; B—basitonic branching; C—mesotonic branching (numbers indicate sequentially developing shoots)

Lateral branching also encompasses sympodial branching (from the Greek syn meaning together). Like monopodial branching, sympodial branching produces a main axis, but it is composed of axes of different orders. The formation of such a main axis occurs as follows. The first-order axis begins to grow, but at some point, apical growth ceases, the growth point dies off, or a flower forms at the apex. As soon as the main axis (the first-order axis) stops elongating and apical dominance is lost, the lateral bud closest to the apex begins to grow, forming the second-order axis. Initially, it forms at an angle to the original axis before taking over its direction. This process repeats multiple times, resulting in a main axis that is actually a system of sympodially linked shoots. This branching pattern means that as the apical Meristems of different-order axes successively die back, A large number of buds (lateral and adventitious) are activated. This leads to a massive expansion of the above-ground Skeletal Structure of the plant and, consequently, a significant increase in its photosynthetic surface area. Sympodial branching is considered the pinnacle of plant branching evolution. It is predominantly characteristic of angiosperms. However, in its pure form, sympodial branching is found only in herbaceous plants. In woody plants, sympodial branching is combined with monopodial branching. In trees, evidence of sympodial branching—in the form of dead-end segments of previous-order shoots—can only be observed on newly growing young twigs. Plants with sympodial branching typically develop a spreading crown (e.g., linden, birch, apple tree).

False-dichotomous (dichasial) branching is another form of lateral branching. In this case, just below the apex of the original shoot (which either dies back or terminates in an inflorescence), two lateral shoots of the next order develop from axillary buds. This type of branching is characteristic of many members of the family Caryophyllaceae. False-dichotomous branching is also found in certain trees (such as horse chestnut and maple) and shrubs (such as lilac and dogwood). In woody plants, false-dichotomous branching, much like sympodial branching, is combined with monopodial branching (Fig. 57).

Fig. 57. Diagrams of false-dichotomous (A) and sympodial (B) types of branching, and The structure of the main axis (trunk) (C) in trees with a sympodial branching pattern:

1–4—sequentially developing shoots; 1a–3a—dying shoot apexes; 5—apical buds; 6—leaf scars

The emergence of the main axis enabled plants to rise above the land surface and move closer to the energy source utilized in Photosynthesis.

4.4. Evolutionary Emergence and Development of Phylloids and Leaves

Branching was not the only mechanism for increasing the surface area of higher plants. While the most primitive fossil plants (such as Cooksonia and Rhynia) carried out photosynthesis via axial body parts that received uneven illumination, more advanced plants developed specialized flattened lateral organs—leaves. These were oriented more or less horizontally, facilitating better light interception and more efficient photosynthesis. The evolution of leaves dramatically expanded the aerial surface area of plants, boosting assimilation, gas exchange, and Transpiration rates while simultaneously elevating their level of biological Organization.

According to prevailing theories, leaves could have evolved through two distinct pathways. The presence of small enations in psilophytes and larger spine-like structures in Asteroxylon suggests that leaves may have originated as superficial outgrowths (enations) of the axial organs. In such outgrowths—phylloids—a vascular strand extended from the conducting System of the mesomes, transporting assimilation products from the phylloid (leaf) to the mesomes and rhizomoids, while also supplying water and dissolved nutrients to the phylloid. Leaves formed via enation are small, herbaceous, or scale-like. They are characteristic of members of the microphyllous evolutionary line (from the Greek micros meaning small and phyllon meaning leaf). Among modern plants, representatives of the microphyllous Lineage include the lycophytes (Fig. 58).

Fig. 58. Evolutionary diagram of enational (microphyllous) leaves (phylloids) formation: A–G—sequential development of phylloids: 1—fragment of a psilophyte mesome; 2—primary cortex; 3—stele Vascular System; 4—initiation of a lateral outgrowth (enation); 5—emergence of the vascular trace from the stele into the enation; 6—completion of the enational leaf and its vascular bundle (single vein)

In contrast to lycophytes, ferns and other taxa of higher spore-bearing and seed plants formed leaves, According to the telome theory,

via cladification (from the Greek klados meaning branch), meaning they are of axial origin. Such leaves likely formed through the fusion, shortening, and flattening of telomes that had lost their sporangia. Cladificational (syntelomic) leaves are characterized by apical growth, which is intrinsic to axial structures. Leaves formed by cladification are relatively large. The leaf's vascular system connects with the plant's axial vascular system, creating leaf gaps (parenchymatous zones that form in the axial vascular tissue just above the point where the leaf trace departs). Cladificational leaves are characteristic of members of the macrophyllous evolutionary line (from the Greek macros meaning large) (Fig. 59).

Fig. 59. Evolutionary diagram of cladificational (syntelomic) leaves formation: A—fragment of a psilophyte; B—fragment of a psilophyte with telomes that have lost their sporangia; C—shortening and fusion of sterile telomes; D—flat leaf: 1—fragment of mesomes; 2—telomes; 3—primary cortex; 4—vascular system of mesomes and telomes; 5—vascular system of the fully formed cladificational leaf