Angiosperms undergo double fertilization. Double fertilization produces a zygote and an endosperm. Afterwards, the ovule developes into a seed and an ovary into a fruit that protects the seed(s).
Growth and Development of Ovule
The ovule grows and developes into a seed. The zygote and endosperm are in it. The zygote continues to grow and develops into an embryo.
Gowth and Development of endosperm
The endosperm grows and develops earlier, compared to the embryo. The endosperm is rich in nutrients and it servers as food storage which is utilized for the growth and development of the embryo. In most monocotyledons,the nutrient reservers is put in the cotyledon before seed grows and developes further.
Gowth and Development of embryo
Growth and development of the embryo is begun with zygotic cleavage by mitosis to produce basal and apical cells. Basal cells develop into a suspensor. The suspensor function as a connector between the embryo and the outer layer of the ovule. It also allows nutrients flow from the mother plant or the endosperm. Apical cells develop into a proembryo. This is followed by embryo`s development. In then develops a stem and roots.
Structure of mature seed
During its maturation, the seed is dehydrated until water content is only about 5% to 15% of its weigth. Then, the embryo stop developing until the seed germinates. A mature seed contains an embryo that is surrounded by a cotyledon, or an endosperm, or both. The seed is protected by a seed coat (testa).
Dicotyledon plants have two cotyledons. In their embryo`s development, epicotyls and hypocotyls ar formed. Epicotyls are located above the cotyledon. The tip of the epicotyls is called the plumule, which is foliage of leaves. Hypocotyls are located below the cotyledon. They end at the radical. Monocots have only a single cotyledon.in some monocots, the cotyledon is called a scutellum. It is thin and the endosperm suppresses where it is positioned. The scutellum absorbs nutrients from the endosperm during germination. The embryos of grass plants are surrounded by coleorhizae and coleoptiles. Coleorhizae protects the radical and the coleoptiles protects leaves and stem primordia
Seed Germination
Seeds will develop under the correct environmental conditions. But other seeds may become dormant which means that the seed stops growing and developing. A seed may become dormant when he environmental conditions do not support germination.
When germination starts, seed dormancy end. The end of dormancy is marked by the entrance of water into the seed, a process called imbbition. This process induces enzymatic activities (biocatalysts that play a role in metabolism) so germination starts. After dormancy ends, the plant cells multiply in number by active division. However, newly formed cells are not differentiated yet. When te clump of cells reaches a certain cell mass, the plant starts the differentiation process.
Differentiation is a process of adding distinct cell types and functions to the organism. Afterward, organs are formed by organogenesis. Organogenesis of many different shapes of organs to complete the structure and function of the organism is called development or morphogenesis.
Seed can germinate because they have an embryo. There are 3 parts of an embryo: radical,cotyledon, and caulicle.
Radicle (Embryonic Root) | Cotyledon (Embryonic Leaf) | Caulicle ( Embryonic Stem) |
The radicle will grow and function into roots. The tip of radicle is facing toward the micropylar endosperm (seed canal). During germination, the root grows through this canal and ruptures the seed coat. | Cotyledon is the first structural leaf of the plant. It has several function : Ø As storage for food. It is thick, one side is usually curved and the other side is flat Ø A structur for photosynthesis Ø To absorb nutrient from endosperm, which form a thin layer called the shield shaped scutellum in monocots. | There are two parts of the caulicle: the epicotyl and hypocotyls. Ø Epicotyl is a stem segment above the cotyledon thet will grow into the stem and leaf. Ø Hypocotyl is a stem segment below the cotyledon that will grow to elongate into roots |
There are two types of germination based on the strating location: epigeal and hypogeal.
Epigeal germination
Epigeal germination is marked by the up-lifting of the hypocotyls above the ground level. Cotyledon divides quickly to form leaves. This type of germination can be observed inmung bean plants( phaseolus radiatus)
Hypogeal germination
Hypogeal germination is marked by the formation of a rudimentary stem that appears above ground level, while the cotyledon stays inside the soil ( hypocotyls stays underground). This process can be observed in pea plants (pisum sativum).
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