PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 9. DEVELOPMENT, STRUCTURE, TYPES OF SEEDS AND FRUits

9.3. Dispersal of Fruits and Seeds

Many plants are monocarpic, meaning they produce fruits and seeds only once during their ontogeny and die shortly thereafter. This group includes all annuals as well as biennials, which yield fruits and seeds just once, exclusively in their second year. Certain perennial tropical plants are also monocarpic; for example, the American agave can live for up to 100 years and dies after flowering and fruiting.

Plants that repeatedly produce fruits and seeds throughout their ontogeny are referred to as polycarpic. This group encompasses woody species and the majority of perennial herbaceous plants.

The significance of fruits extends far beyond their role in the life of the plants themselves. For many animal groups, fruits serve as a primary food source, and a range of morphophysiological traits in both the animals and the fruits they consume is the result of long-term coevolution. Plant fruits significantly determine the nature of trophic relationships within a biocoenosis.

Fruits and seeds primarily ensure the dissemination and spatial spread of plants. There are two main modes of seed dispersal (dissemination). One relies on mechanisms that evolved through the adaptive evolution of the plants themselves, while the other utilizes external factors such as water, wind, animals, and humans. The first mode of dissemination is called autochory (from the Greek auto meaning self and choreo meaning to spread), and the second is termed allochory (from the Greek allo meaning other and choreo meaning to spread). Plants utilizing these respective mechanisms are known as autochores or allochores (Fig. 217). The fruits and seeds of autochores are typically dispersed close to the parent plant. In these cases, seeds are either actively expelled from the fruits or fall passively to the ground, sometimes together with the fruit. In many plants with dry dehiscent fruits, seed dispersal occurs through the rapid opening of the fruit and the twisting of its valves, as seen in certain legumes (such as beans, peashrub, vetch) and touch-me-nots. In violets, the capsule valves curve inward, effectively squeezing the seeds out. The squirting cucumber (Ecballium elaterium) actively ejects its seeds: under high hydrostatic pressure, its fruits forcefully discharge a jet of mucilaginous fluid containing the seeds. Heavy fruits and seeds simply drop near the parent plant, which is characteristic of oak acorns, walnuts, palms, horse chestnuts, and others. A unique mechanism is found in the peanut or groundnut (Arachis hypogaea), whose developing fruits bury themselves in the soil to mature underground.

Fig. 217. Dispersal of seeds (A–C) and fruits (D–F) in various angiosperm representatives: A—active expulsion of seeds from the berry-like fruit of the squirting cucumber (Ecballium elaterium); B—active ejection of seeds from the capsule of the touch-me-not (Impatiens noli-tangere); C—anemochorous seeds with pappus-like tufts in fireweed (Chamaenerion angustifolium); D—anemochorous winged nutlet of silver birch (Betula pendula); E—zoochorous achene with barbed hooks in nodding bur-marigold (Bidens tripartita); F—anemochorous achenes with pappus in common dandelion (Taraxacum officinale)

In some plants, mature fruits possess the ability to self-bury (such as feather grass and stork's-bill). They feature awns, which are elongated pistil styles that are spirally twisted at the base and straight at the top. When moistened, the hygroscopic spiral part unfurls, screwing the fruit into the soil, while backward-pointing hairs prevent it from slipping back out.

Allochory is manifested in four main ways: anemochory, hydrophory, zoochory, and anthropochory. The fruits and seeds of anemochores (from the Greek anemos meaning wind and choreo meaning to spread) are transported by air currents. Some produce extremely tiny, dust-like seeds (such as wintergreens and orchids), while others have evolved various flying adaptations. For instance, lindens use winged bracts for this purpose, whereas ashes, maples, birches, and elms feature winged fruit appendages. Willows, poplars, milkweeds, and fireweeds develop hair tufts on their seeds. The plumed seed heads of dandelions, sow thistles, thistles, and salsify are widely known. Desert habitats are characterized by tumbleweed plants: once detached from their roots, they roll across the ground, scattering seeds along the way. Examples include spreading baby's breath from the Caryophyllaceae family and saltwort from the Chenopodiaceae family.

The fruits of hydrophores (from the Greek hydro meaning water and choreo meaning to spread) possess adaptations that protect seeds and fruits from wetting and provide buoyancy, typically through air-filled cavities, corky tissue bands, and the like. The most famous tropical hydrophore is the coconut palm, which has colonized the tropical coasts of all oceans. Most aquatic and wetland plants in our flora—such as water plantain, water hemlock, hornwort, pondweed, and many others—are also hydrophores. In addition to standing and running water, rainwater plays a role in fruit and seed dispersal, which is especially important for plants growing on mountain and hill slopes.

The dispersal of fruits and seeds via animals is called zoochory (from the Greek zoon meaning animal and choreo meaning to spread). Succulent, brightly colored fruits attract mammals and birds and are consumed by them. The seeds within pass undamaged through the digestive tract or are regurgitated by the animals far from where they were ingested. The variety of fruit colors, tastes, and aromas that makes them attractive is clear evidence of plant-animal coevolution. Fruit and seed dispersal is also facilitated by various appendages such as hooks and barbs (found in beggarticks, agrimony, bedstraw, hound's-tongue, etc.), as well as sticky mucilaginous outer seed coats (species of plantains, twinflower). The most active seed carriers are birds (ornithochory), insects—particularly ants (myrmecochory)—and mammals.

Anthropochory (from the Greek anthropos meaning human and choreo meaning to spread) refers to dispersal associated with human activity. The development of transportation routes, including transcontinental ones, has greatly facilitated the long-distance translocation of plants. For example, Elodea and thistles made their way to Europe, while plantain reached the American continent. All weed species are anthropochores. Cultivated plants, which are introduced and farmed across vast areas of every continent, are also closely related to anthropochores.

Thus, the tremendous diversity of dissemination methods reflects the multitude of adaptive traits and properties of plants, enabling them to occupy various ecological niches.

9.4. Seed Germination and Seedling Development

Following their formation, seeds enter a dormant state. Seed dormancy can be either enforced or innate (organic). In enforced dormancy, seeds fail to germinate simply because suitable environmental conditions (such as specific soil moisture, temperature, atmospheric gas composition, or soil acidity) are lacking.

An innate dormancy state is characteristic of the overwhelming majority of wild plants and many cultivated crops. Such seeds are incapable of germinating or exhibit reduced germination rates even under conditions otherwise favorable for growth. The ability of seeds to remain in innate dormancy for a certain period is an adaptive mechanism ensuring species survival. Innate dormancy helps preserve seeds in the soil during conditions unfavorable for development and allows them to accumulate in the soil (forming a seed bank). The duration of innate dormancy varies among species, lasting from 1 to 2 years or even longer.

Upon receiving moisture, optimal temperature, and good aeration, a seed breaks dormancy and germinates. This happens most readily in the seeds of annual plants. For other seeds and normal seedling development, cold stratification is mandatory—a prolonged exposure of seeds to low temperatures in a moist environment with adequate aeration. This is how the seeds of stone and pome fruits, many woody plants, and ginseng germinate. There is also a group of hard-seeded plants whose seeds germinate only after scarification, which involves disrupting the integrity of the seed coat. In nature, this occurs due to extreme temperature fluctuations or when water rolls seeds along a stony riverbed; under artificial conditions, seeds are rubbed with sand, scalded with boiling water, or their seed coats are nicked (as in some lupine species).

By absorbing water, the seed swells significantly, leading to the activation of existing enzymes and the synthesis of new ones that promote nutrient hydrolysis. All of this lays the groundwork for embryo development and seedling formation.

A seedling is a young plant possessing its first pair of true leaves, characterized by a specific type of nutrition and respiration.

During early germination stages, respiration may be entirely anaerobic, but as soon as the seed coat ruptures, it shifts to aerobic respiration, requiring an oxygen supply. If the soil is waterlogged, the oxygen level may prove insufficient for this type of respiration, making germination impossible.

Although many seeds germinate across a relatively wide temperature range, upper and lower limits specific to each species typically exist. The minimum temperature for many plants corresponds to 0–5 °C, the maximum to 45–48 °C, and the optimum to 25–30 °C.

As a rule, the embryonic root (radicle) is the first to begin growth. It breaks through the seed coat near the micropyle and penetrates the soil, ensuring the uptake of water and mineral salts into the plant. The shoot emerges later. The pattern of seed germination determines seedling morphology. For instance, in dicots, epigeal (above-ground) and hypogeal (underground) types of germination are distinguished. In beans, lupines, tomatoes, cucumbers, and others, once the root appears, the hypocotyl—or sub-cotyledonary stem (the stem section from the root collar to the cotyledons)—elongates and bends into a loop. As a result, the delicate shoot apex is pulled rather than pushed through the soil, avoiding damage. When the curved hypocotyl reaches the soil surface, it straightens and lifts the cotyledons above ground, which is why this is called epigeal germination (Fig. 218). The cotyledons, which were appressed in the seed, unfold, and the plumule enclosed between them develops into a shoot with true green leaves.

Fig. 218. Epigeal seed germination: A—common bean (Phaseolus vulgaris); B—common sunflower (Helianthus annuus): 1—seed coat; 2—hypocotyl; 3—taproot; 4—lateral roots; 5—cotyledons; 6—epicotyl; 7—first true leaves

The nutrient reserves stored in the cotyledons are gradually depleted, causing the cotyledons to become less massive, turn green, and temporarily perform the function of typical leaves. Eventually, they wither and drop off, while the seedling becomes an independent, photosynthesizing, autotrophic organism. In some plants, the cotyledonary leaves also play a hormonal role by regulating the plant's reproductive activity. Early damage or removal of the cotyledons can reduce fruiting or lead to complete sterility in the plant.

Hypogeal, or subterranean, germination is driven by the active growth of the epicotyl. The epicotyl (the internode above the cotyledons) is the section of the stem located between the cotyledons and the first true leaf. It elongates to form a loop that pushes the plumule up to the soil surface. The first two leaves on the stem are typically underdeveloped, while typical plant leaves are positioned above them. Since the hypocotyl does not elongate, the cotyledons remain in the soil, where they eventually break down. This type of germination occurs in peas, broad beans, chickpeas, lentils, and oaks (Fig. 219).

Fig. 219. Hypogeal seed germination: A—garden pea (Pisum sativum); B—English oak (Quercus robur): 1—seed coat; 2—hypocotyl; 3—taproot; 4—lateral roots; 5—cotyledons; 6—acorn fruit; 7—epicotyl; 8—true leaves

Epigeal germination is typical for most dicotyledonous plants. In grasses, as in dicots, the embryonic root is the first to form during germination, but its tip is protected by the coleorhiza (root sheath) rather than a root cap. In seedlings of rye, wheat, barley, and oats, several roots emerge simultaneously; these are termed primary seminal roots and represent very early-formed adventitious roots (Fig. 220). The germination of a corn caryopsis, however, produces a single taproot. Following root emergence, the aerial shoot develops from the plumule. It is protected by the coleoptile, the first closed leaf of the plumule, whose cone-shaped, pointed tip easily penetrates soil particles. The coleoptile is whitish in color (purple in rye) and is often referred to as the "feather." In addition to its protective function, the coleoptile also plays a hormonal role.

Fig. 220. Development of cereal seedlings: A—common wheat (Triticum aestivum); B—barley (Hordeum vulgare); C—maize (Zea mays): 1—naked caryopsis; 2—caryopsis covered with floral glumes (hulled); 3—caryopsis coat; 4—primary seminal roots; 5—taproot; 6—coleorhiza; 7—coleoptile; 8—emerging true leaves

In naked caryopses, the coleoptile immediately grows upward. In hulled grasses such as barley and oats, the coleoptile initially grows beneath the glumes along the caryopsis before emerging from the opposite end. Soon after reaching the soil surface (on the 5th–7th day), the tip of the coleoptile ruptures, and the first true leaf emerges.

The period from seed germination to the transformation of the seedling into an independent organism is the most critical stage in plant ontogeny, during which it is most vulnerable to various biotic and abiotic environmental factors.