SEEDS

SEED STRUCTURE

Remember that ovules develop into seeds. The concave tide of an ordinary kidney bean (the seed of a dicot plam) has small, white scar called the Hilum. The hilum marks the point at which the ovule was attached to the ovary wall. A tiny pore called the micropyle is located next to the hulom. If this bean is placed in water for an hour or two, it may swell nough to split the seed coat. Once the seed coat is removed. the two halves, called cotyledons. The cotyledons, which have a tiny, immature plantlet along one edge between them, are food-storage organs that also function as the "seed leaves" of the seedling plant. The cotyledons, and the tiny, rudimentary bean plant to which they are attached, constitute the embryo. Some seeds (eg, those of grasses and all other monocots) have only one cotyledon. The tiny embryo plantlet has undeveloped leaves and meristem at the upper end of the embryo axis. This embryo shoot is called a plumule. The cotyledons are attached just below the plumule. The very short part of the stem the cotyledon is called the epicotyl, while the stets below the anachment point is the hypocotyl. In an embryo, it is often difficult to tell where the stem ends and the root begins, but the tip that will develop intoo a root is called radicle. 

SEED FORMATION AND DEVELOPMENT

Seed Formation

A seed is formed when fertilised ovule divides by mitosis. It stores food and has the potential to develop into a new plant under optimal conditions. Fertilization is the process of fusion of male gamete and female gamete to form a zygote. Pollen grains are transferred to stigma by various pollinating agents such as water, wind, butterflies, insects, animals, birds, etc. After reaching stigma, the male gamete fuses with the egg in the ovule and forms a zygote. Thus fertilization takes place and so the formed zygote divides and develops into an embryo. Following the fertilization, every part of the flower sheds off except the ovary. The ovary of the flower develops into the fruit while ovules develop into seeds. The formation of seed completes the process of reproduction in plants. Within the seed, the growing embryo develops and matures. 

Seed Development

Seed development is a complex process that commences after double fertilization. Both forward and reverse genetic studies have revealed the critical roles of phytohormones in regulating seed development and the associated agronomic traits. The growing evidence points to the complex interactions among underlying genetic pathways due to hormone cross talk or shared signaling components. Moving forward to deconvolute these complex interactions requires an in-depth understanding of the genes regulating individual hormone pathways. Here, we summarize the multifaceted roles of key genes regulating biosynthesis and signaling of plant hormones, and the broad spectrum of mechanisms underpinning hormone action during seed development. The gain- and loss-of-function phenotypes associated with agronomically important seed traits, namely, seed size, weight, shape, number, longevity, and dormancy, provide compelling evidence for the plant hormones as crucial metabolic engineering targets to optimize seed traits in crop plants.

TYPES OF SEEDS

There are two types of seeds associated with flowering plants. Dicotyledons and monocotyledons. 

Monocotyledonous Seeds: Seeds having a single cotyledons. E.g rice, orchid, maize, etc. 

Dicotyledonous Seeds: Seeds having two cotyledons. E.g pea, beans, mango, mustard, etc. 

SEED DISPERSAL MECHANISM

Why are so many species of orchids rare, while dandelions, shepherd's purse, and other weeds occur all over the world? Why do some plants occupy entire mountain ranges, while others are confined to small niches occupying less than a hectare (2.47 acres) of land? 

The answers to these questions involve many different factors, including climate, soil, the adaptability of the plant, and its means of seed dispersal. It is important for seeds to be carried away from the mother plant before they germinate. This prevents competition with the mother plant and avoids inbreeding. The following sections describe several effective seed dispersal mechanisms.

Wind Dispersal (Anemochory)

Plants have developed various adaptations for wind dispersal, such as samaras with curved wings and seeds enclosed in inflated sacs. From tiny orchid seeds to dandelion fruitlets with plumes, wind dispersal mechanisms ensure seeds are carried away from the parent plant, promoting genetic diversity and effective distribution over long distances. 

Animal Dispersal (Zoochory)

Animal dispersal occurs when animals, such as Birds, mammals, and ants unintentionally and intentionally carry seeds away from the parent plant through ingestion, adhesion, and storage.

Water Dispersal (Hydrochory)

Plants have evolved adaptations for water dispersal, including seeds with trapped air and spongy pericarps, enabling them to survive and colonize new habitats through water bodies and ocean currents. Water dispersal mechanisms allow seeds to travel long distances, ensuring plant survival and distribution across diverse environments. The seeds of the plant that grow in or near water bodies are dispersed through the water. 

Other Dispersal Mechanisms and Agents

Plants employ diverse mechanisms like explosive seed ejection and adhesive fruits for dispersal. From mechanical seed ejection in witch hazel to ants carrying seeds to their nests, plants utilize various strategies for effective seed dispersal. Human activities also play a significant role in seed transport, unintentionally spreading plant species across continents and necessitating regulations to prevent the spread of diseases and pests. 

Ballistic Dispersal (ballochory)

Mode of dispersal by which the diaspores are actively or passively catapulted away from the plant. Ex. Witch Hazel (Hamamelis virginiana), Dwarf mistletoes (Arceuthobium minutissimum), Manroots (Marah fabaceus).

Human Dispersal (anthropochory)

Humans, both intentionally and unintentionally, transporters of fruits and seeds. Human Activities like agriculture, hoticulture and travel.

GERMINATION AND SEEDLING GROWTH

Germination, the initial stage of a seed's growth, is a complex process influenced by both internal and external factors. For a seed to germinate, it must be viable (alive) and have overcome any dormancy periods. Dormancy, a state of suspended development, can be caused by various factors like mechanical barriers (thick seed coats), physiological inhibitors (chemicals within the seed), or both. 

Breaking Dormancy: 

• Mechanical Methods: Scarification, a process of artificially breaking the seed coat, can be achieved through methods like abrasion with sandpaper or soaking in hot water. This mimics natural processes like soil erosion or freezing-thawing cycles. 

• Physiological Methods: Seeds containing growth inhibitors can be treated by soaking them in water to leach out the inhibitors. - After-Ripening: Some seeds, like those of American holly, require a period of after-ripening, where the embryo undergoes further development, even after the seed has been released from the fruit. 

Environmental Factors: 

• Water: Water is essential for germination, as it rehydrates the seed and allows for the activation of enzymes. Some seeds have specialized appendages to absorb water more efficiently. 

• Oxygen: Oxygen is crucial for respiration, the process that provides energy for the growing embryo. Waterlogged conditions can reduce oxygen availability and hinder germination. 

• Temperature: Most seeds have an optimal temperature range for germination, typically above freezing but below 45°C. 

• Light: The role of light can vary depending on the plant species. Some seeds require light to germinate, while others germinate only in darkness. 

Growth

Once germination is complete, the seedling continues to grow, developing roots, stems, and leaves. This growth is fueled by photosynthesis, a process where the plant converts sunlight into energy. The roots anchor the plant and absorb water and nutrients from the soil, while the stems support the leaves and transport nutrients throughout the plant. Leaves capture sunlight for photosynthesis. 

ECONOMIC AND ECOLOGICAL IMPORTANCE

The term economic is used to describe the importance that individuals place on ecosystems, which includes not only the income generated from using ecosystem goods and services, but also other benefits that ecosystems provide for human welfare. Ecological and Economics addresses the relationships between ecosystems and economic systems in the broadest sense. These relationships are the locus of many of our most pressing current problems.