Epigeal Germination in a mung bean And Hypogeal Germination In Pea

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 the environmental conditions do not support germination. 

When germination starts, seed dormancy ends. The end of dormancy is marked by the entrance of water into the seed, a process called imbibition. 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. ia newly formed cells are not differentiated yet. When  the 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. When leaves are formed, the plant can carry out photosynthesis, this is a process of harnessing the sun’s energy to convert carbon dioxide into glucose. Glucose is then used as energy source for growth and development. 

Seeds can germinate because they have an embryo. There are three parts of an embryo: radicle, cotyledon, and caulicle.

Radicle (Embryonic Root)

The radicle will grow and function into roots. The tip of the radicle is facing toward  the micropylar endosperm (seed canal). During germination, the root grows through this canal and ruptures the seed coat.

Cotyledon (Embryonic Leaf)

Cotyledon is the first structural leaf of the plant. It has several functions:

1. As storage for food. It is thick, one side is usually curved and the other side is flat;
2. A structure for photosynthesis;
3. To absorb nutrient from the endosperm, which form a thin layer called the shieldshaped scutellum in monocots.

Caulicle (Embryonic Stem)

There are two parts of the caulicle: the epicotyl and hypocotyl. The epicotyl is a stem segment above the cotyledon that will grow into the stem and leaves. The hypocotyl is a stem segment below the cotyledon that will grow to elongate into roots.

There are two types of germination based on the starting location: epigeal and hypogeal.

Epigeal Germination

Epigeal germination is marked by the up-lifting of the hypocotyl above the ground level. Cotyledon divides quickly to form leaves. This type of germination can be observed in mung bean plants (Phaseolus radiatus). Observe Figure 1.5a.

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 (hypocotyl stays underground). This process can be observed in pea plants (Pisum sativum). Observe Figure 1.5b.

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