Seeds: Videos & Practice Problems

The transformation of a small seed into a towering tree begins with the process of fertilization, where pollen grains deliver sperm to the ovule within a flower. This leads to embryogenesis, the development of the seed and the plant embryo. Following fertilization, the zygote forms, which subsequently divides into two daughter cells: the apical cell and the basal cell. The apical cell is crucial as it develops into a mass of cells that will differentiate into the primary meristems: the protoderm, ground meristem, and procambium, ultimately forming the plant embryo.

The basal cell, on the other hand, develops into a structure known as the suspensor, which provides support for the embryo, although only one cell from the suspensor contributes to the embryo itself. Within the embryonic plant, there are cotyledons, which are embryonic leaves. Plants are categorized based on the number of cotyledons they possess: monocots have one cotyledon (from the prefix "mono," meaning one), while eudicots have two cotyledons (with "eu" indicating true, thus referring to two true leaves).

Additionally, the embryonic plant includes the hypocotyl, which serves as the embryonic stem, and the radical, which is the embryonic root. The distinction between monocot and eudicot seeds can be observed through their cotyledons: monocots feature a single cotyledon, while eudicots have two. Other important seed structures include the seed coat, which protects the seed, and the endosperm, which provides essential nutrients for the developing embryo. Some plants also possess an epicotyl, an extension of the embryonic stem that grows above the cotyledons, becoming more visible as the plant sprouts.

Germination is the critical process through which a plant develops from a seed, initiated when the seed absorbs water in a process called imbibition. The seed coat, a protective outer layer formed from the integument of the ovule, plays a significant role in this process. While some seed coats can swell and break open due to imbibition, others may be too thick and require physical penetration, such as being digested by animals, to allow germination.

The sprouting mechanism varies between monocots and eudicots. Eudicots feature a hypocotyl that curves upward, pushing the cotyledons out of the soil. This arching movement allows the plant to emerge effectively. In contrast, monocots push their shoots straight up through the soil, necessitating protective structures known as the coleorhiza and coleoptile. The coleorhiza emerges first, safeguarding the radical (the embryonic root), while the coleoptile protects the cotyledons. This distinction in growth patterns highlights the adaptations of each group to their environments.

Seed dispersal, the process of transporting seeds away from the parent plant, occurs through various mechanisms, including wind, water, and animals. For instance, dandelion seeds have tufts that facilitate wind dispersal, while animals can carry seeds attached to their fur, as seen with burrs. Upon landing in suitable soil, seeds may enter a state of dormancy, delaying germination until conditions are optimal. Some seeds can remain dormant for years, waiting for the right moment to sprout.

Fruits play a crucial role in the dispersal of seeds, serving as enticing structures that attract animals. These seed-bearing structures develop from the ovary of a flower after it has bloomed. For instance, in a peach, the seed is encased within the fruit, which consists of several key components: the endosperm, embryo, and seed coat. Surrounding the seed is the pericarp, the edible part of the fruit that ripens from the ovary, providing a sweet coating that appeals to animals.

Fruits come in various shapes and sizes, and some commonly referred to as vegetables are technically fruits. A prime example is zucchini, which, despite its savory flavor, is classified as a fruit because it develops from the ripened ovarian walls surrounding seeds.

Fruits can be categorized based on their development. Simple fruits, like cherries, arise from a single flower with one or more fused carpels, resulting in a straightforward structure with a single seed. In contrast, aggregate fruits, such as raspberries, develop from a single flower with multiple carpels, each containing its own seed. Multiple fruits, like pineapples, form from clusters of flowers, while accessory fruits, such as strawberries, develop from both ovary tissues and surrounding tissues, with seeds located on the outside.

This diversity in fruit types illustrates the various developmental processes that occur in flowering plants, highlighting the intricate relationship between flowers and the fruits they produce. Understanding these classifications enhances our appreciation of plant biology and the role of fruits in ecosystems.