BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

ECOLOGICAL GROUPS OF PLANTS

Ecological groups of plants according to their Water requirements

Based on their water requirements, plants are divided into various ecological groups. The main ones are hydatophytes, aerohydatophytes, hydrophytes, hygrophytes, mesophytes, and xerophytes. Intermediate forms exist between these groups.

Hydatophytes are aquatic plants that are entirely or almost entirely submerged in water (e.g., Elodea, Vallisneria). Their leaves are thin, often dissected, contain chlorophyll, and lack a cuticle. The ROOT System is heavily reduced or absent. A characteristic feature of these plants is aerenchyma with numerous air-filled intercellular spaces. Mechanical and conducting Tissues are poorly developed. Water and mineral salts are absorbed across the entire surface of the plant. Hydatophytes readily reproduce vegetatively.

Aerohydatophytes are hydatophytes in which some or all leaves float on the water surface (e.g., yellow water-lily, white water-lily, common duckweed). In terms of structural features, they resemble hydatophytes. Their distinguishing characteristic is floating leaves with a well-developed palisade parenchyma, much like typical sun leaves. The intercellular space system is also well-expressed, and Stomata are located on the upper side of the leaves. In yellow water-lilies, there are up to 650 stomata per 1 mm of leaf surface.

Hydrophytes are semi-aquatic plants partially submerged in water, common along the shores of water bodies and in swamps (e.g., common reed, cattail, arrowhead). They possess aerenchyma along with well-developed mechanical and conducting tissues. Many of them (such as the arrowhead) exhibit pronounced heterophylly. The epidermis of hydrophytes contains numerous stomata, resulting in very high Transpiration rates.

Hygrophytes are terrestrial plants growing in conditions of high atmospheric humidity and on damp soils (e.g., sundew, Touch-me-not). Their leaves are often thin, with a poorly developed cuticle, hydathodes (water stomata), and intercellular spaces. Tissue water content can reach up to 80%. Hygrophytes are highly sensitive to decreases in humidity, and even a minor drought can cause wilting and plant death.

Mesophytes are plants of moderately moist habitats. These include species of broad-leaved deciduous forests, floodplain meadows, and most cultivated plants in gardens, orchards, and fields. Typical mesophytes include red clover, meadow bluegrass, littleleaf linden, and European hornbeam. The leaf mesophyll Cells are small, with few stomata and Veins, and an osmotic pressure ranging from 20-25 atm.

Xerophytes are plants of dry, open habitats, characteristic of arid steppes and deserts. Under unfavorable conditions, they halt growth and enter a state of depression. These plants are characterized by xeromorphism—a set of morpho-Anatomical Features aimed at survival under water scarcity. Most xerophytes possess deep root systems. For instance, camel thorn roots can absorb water from a depth of 10-20 m, and alfalfa roots from 6-8 m. In some xerophytes, the root systems are shallow, highly branched, and often temporary—ephemeral roots that utilize water from brief rains. The roots and stems of many xerophytic shrubs are covered with cork, which protects them from desiccation. A characteristic feature of xerophytes is the high Osmotic Pressure of their Cell sap, which ensures enhanced suction force and reduces water loss.

Ecological groups of plants according to their light requirements

All plant life processes are significantly influenced by the intensity and quality of light, as well as the duration of daily illumination. Based on light requirements, three ecological groups are distinguished: heliophytes, facultative heliophytes, and sciophytes.

Heliophytes are sun-loving plants that develop best under full illumination and cannot tolerate prolonged shading. These include steppe, meadow, and field plants, such as yellow sweet clover, coltsfoot, feather grass, fireweed, and among trees—larch and black locust. Ephemerals and ephemeroids also belong to heliophytes. Light is the primary limiting factor for forest ephemeroids, which develop, vegetate, and bloom before the trees leaf out. These include snowdrops, Siberian squills, yellow star-of-Bethlehem, and corydalis. Heliophyte leaves exhibit features of xerophytic Structure: thick, rigid, often dissected leaf blades capable of orienting themselves edge-on or at an angle to the sun; well-developed mechanical tissues and palisade parenchyma; a thick cuticle layer over the epidermis; and numerous stomata. The plants have short internodes and shoots, pubescence, and often rosetted leaves. Photosynthesis proceeds intensely within them.

Facultative (shade-tolerant) heliophytes are species that can live under full sunlight but also withstand light shading. These include certain meadow and forest plants: lily of the valley, ground elder, lesser butterfly-orchid, staghorn clubmoss, bird cherry, oak, hornbeam, and linden. Leaf mosaicism is frequently well-expressed in most of them, along with heterophylly and anisophylly in some. Depending on lighting conditions, all these plants can function either as sciophytes with shade leaves or as heliophytes with sun leaves. The bilberry, typically shade-tolerant in forests, becomes a heliophyte in the mountains and tundra.

Sciophytes, also known as skiophytes or umbrophytes, are shade-loving plants that grow in heavy shade under diffuse light and never occupy open areas. They are found in caves, deep water bodies, and the lower strata of shaded forests. They can grow under minimal illumination: mosses and clubmosses at 0.1-0.2% of full daylight; clubmosses and ferns at 0.25-0.5%; angiosperms at 0.5-1.0%. Among broad-leaved forest species, sciophytes include European wild ginger, mercury, and herb Paris; among coniferous forest species, common wood sorrel, etc. Sciophytes frequently develop creeping or trailing shoots or long-lived stolons (e.g., ground ivy, yellow archangel).

A particularly large variety of plant morphological types is observed in tropical forests. These include epiphytes and lianas, which are capable of elevating the bulk of their vegetative and Generative Organs into the upper, well-lit forest strata.

Epiphytes are plants that settle on the trunks and branches of other plants and obtain nutrients from the surrounding environment. Some of them belong to heliophytes, others to sciophytes, but all possess specialized adaptations for capturing water. For example, orchids develop a specialized spongy tissue on their roots called velamen, which rapidly absorbs water; bromeliads form a funnel with their leaf rosettes to collect rainwater; and in Spanish moss from Central America, atmospheric moisture is absorbed by tiny gray scales densely covering the stems and leaves. Among the epiphytes of rain forests are epiphylls, which settle on tree leaves (Lichens, mosses, Algae).

Epiphytes are found in various taxonomic groups of plants, even within the cactus family. Epiphytes are very rare in temperate latitudes.

Lianas are plants with long, climbing, clinging, or twining stems that use support to grow upward toward the light. Lianas can be woody or herbaceous, utilizing tendrils (fabaceous, cucurbit, grape families), rootlets/holdfasts (ivy), prickles (roses), or thorns (blackberries). Familiar twining lianas include hops and field bindweed. Lianas predominantly have elongated internodes, and stem flexibility is ensured by the presence of broad parenchymatous rays between the vascular bundles. Over 2,000 species of lianas grow exclusively in tropical rain forests.

Ecological groups of plants according to their soil requirements

Depending on their need for soil trophic elements, plants are divided into oligotrophic, mesotrophic, and eutrophic. Oligotrophs, or oligotrophic plants, grow on substrates poor in mineral salts, which are mostly acidic. They are widespread in dry pine forests, heathlands, and Sphagnum bogs (e.g., crowberry, heather, cranberry, bog blueberry). Since xeromorphism is characteristic of most oligotrophs, they are termed xeromorphic oligotrophs. Cushion plants can also be classified as oligotrophs. They are low-growing, highly branched, with shoots tightly pressed against one another. Upward growth is inhibited by bright light, winds, and low temperatures. These are plants from various families (Apiaceae, Caryophyllaceae, Rosaceae, Fabaceae, Primulaceae) found in tundras, alpine zones, and oceanic rocky islands.

Mesotrophs, or mesotrophic plants, have moderate requirements for nutrient content in the soil. They occupy an intermediate position between oligotrophs and eutrophs. These include species of coniferous forests, meadows, and fields: bilberry, lingonberry, wood sorrel, yarrow, etc.

Eutrophians, or eutrophic plants, require fertile soils. They thrive on soils rich in humus and mineral salts. These include plants of eutrophic fens, chernozem steppes, broad-leaved forests, and almost all cultivated plants. Notable Examples are common ash-leaved maple/pedunculate oak, ash, ground elder, lungwort, and marsh marigold.

Plants also differ in their requirements for specific elements. For instance, certain plants (hops, raspberries, stinging nettles) require high levels of soil nitrogen for normal GROWTH AND DEVELOPMENT; they are called nitrophiles or nitrogen-loving plants. Based on calcium requirements, calciphiles, calcifuges, and indifferent species are distinguished. Calciphiles are plants that develop well on calcium-rich soils, in areas where limestone, marl, or chalk outcrops occur. Examples include chalk pine, lady's slipper orchids, and chalk flax. Calcifuges (calciphobes), by contrast, avoid alkaline and calcareous soils (Sphagnum mosses, cranberries, matgrass). Indifferent species can grow on soils with any lime content.

Reaction to soil acidity is a major factor for many plants. The state of the soil solution is determined by the concentration of free H⁺ and OH^- ions, characterized by the pH value, and ranges from 3.5 to 9.0. A pH of 7 characterizes neutral soils, pH>7 alkaline soils, and pH<7 acidic soils. Accordingly, plant species are categorized by their pH response into acidophiles, basiphiles, neutrophiles, and indifferent species.

Acidophiles grow on acidic soils. On very acidic soils (pH 3.5–5.0), species such as cranberry, sheathed cottongrass, and marsh Labrador tea are found; on mildly acidic soils, wavy Hair-grass, marsh marigold, and wood anemone thrive. Basiphiles grow on alkaline soils, including red clover, timothy grass, and black locust. Neutrophiles inhabit neutral soils, such as mountain clover, orchard grass, and early sedge. Indifferent species (like lily of the valley, fescue, and herb-paris) grow across soils with varying pH levels.

A fascinating aspect is how plants adapt to saline soils (in salt flats, solonetz soils, saline steppes, and meadows) and brackish water (along seacoasts). Plants growing on saline soils are known as halophytes. Plant adaptation to salinity can vary depending on The Nature of the salts present. For instance, under chloride salinity, plants often become fleshy and succulent (succulents). Some halophytes shed their leaves to minimize transpiration and eliminate excess salt (such as the sea aster).

Halophytes are categorized into three ecological groups: euhalophytes, crinohalophytes, and glycohalophytes. The ecological group opposite to halophytes is glycophytes (or glucophytes), which comprise plants of non-saline soils and freshwater bodies—including meso-, hydro-, hygrophites, and most xerophytes. However, drawing a sharp line between halophytes and glycophytes is impossible, as nature presents a continuous spectrum of intermediate forms.

Ecological Groups of Plants by Mode of Nutrition

Depending on their mode of nutrition, higher plants are divided into autotrophic organisms, which combine photosynthesis with soil nutrition, and heterotrophic organisms, capable of feeding on dead organic matter (Fungi and Bacteria). However, higher (autotrophic) plants also possess various adaptations to utilize not only minerals but also organic substances from the substrate, which occurs in epiphytic lifestyles or on nutrient-poor soils.

Photosynthesizing higher plants often obtain additional nitrogen compounds through Symbiosis with fungi (mycorrhiza) or bacteria (bacteriorhiza) located in their roots. These plants are called symbiotrophs or symbiotrophic plants. Symbiotrophs can be either facultative or obligate. In the former case, both organisms can live independently, whereas in the latter, independent survival is impossible for either Organism. Obligate symbiotrophs include members of the orchid and heath families, whose seeds cannot develop seedlings without fungal symbiosis.

Saprophytes are mycorrhizal symbiotrophs in which the higher plant has lost chlorophyll and the capacity for photosynthesis. Such plants are whitish, brown, or pinkish, lacking leaves and bearing only scale-like structures along with thick, fleshy roots colonized by fungi. Examples include the bird's-nest orchid, coralroot, and pinedrop.

Parasites and Hemiparasites are plants that live entirely or partially at the expense of other plants. Obligate parasites lack chlorophyll, feature heavily reduced stems and leaves, and possess haustoria (sucking roots) instead of conventional roots. Examples include dodder (which parasitizes various plants), toothwort (living on hazel roots), and broomrape (which colonizes the roots of cultivated and wild plant species). Hemiparasites are capable of independent assimilation; they feature normal green shoots with leaves, alongside conventional roots or modified root-like suckers. Hemiparasites include mistletoe, cow-wheat, yellow-rattle, and others.

Insectivorous Plants perform photosynthesis independently, yet they can also capture and partially digest insects using Proteolytic Enzymes and organic acids. This mechanism compensates for a deficiency of nitrogen and other trophic elements in their substrate. They are typically found in forests, bogs, and wetlands, predominantly in tropical regions. There are about 500 species belonging to the families Droseraceae, Nepenthaceae, and Lentibulariaceae. Such plants feature diverse trapping mechanisms formed mostly from modified leaves. For instance, in Nepenthes, the upper part of the leaf resembles a pitcher, from whose slippery rims insects fall inside. In sundews (Drosera), leaf blades are covered with red glandular hairs where insects stick and are subsequently digested within the curled leaf by sticky secretions. In bladderworts (Utricularia), the leaves form tiny bladders with inward-opening Valves, trapping daphnia which are then digested by enzyme-rich fluids.