Types of germination of seeds will highlight the three main types of seed germination cycle: hypogeal germination, epigeal germination, vivipary (viviparous germination).
Types of germination of seeds
The cotyledons do not emerge from the soil surface during this type of germination. The epicotyl (a part of the embryonic axis between the plumule and the cotyledons) elongates in such seeds, pushing the plumule out of the soil. All monocotyledons germinate hypogeally. Gram, pea, and groundnut are common examples of hypogeal seed germination cycle in dicotyledons.
Radicle and plumule emerge from monocotyledons (e.g., wheat, maize, rice, coconut) by piercing the coleorrhiza and coleoptile, respectively. The plumule develops upward, and the first leaf emerges from the coleoptile. The radicle forms the primary root, which is quickly replaced by a network of fibrous roots.
Germination of Pea Seed
The seed absorbs water and expands. The radicle emerges and first penetrates the soil, where it forms a root system by sprouting secondary branches. The epicotyls are the first to grow. It forms an arch and raises the plumule above ground. The aerial shoot is quickly formed by the plumule. The cotyledons remain in the soil the entire time.
Germination of Maize Grain
Successive Stages of hypogeal seed germination cycle of fruit of maize.
The grain absorbs water from the moist soil. The coleorhiza appears as a shining knob after piercing the base of the caryopsis (fruit). After a while, the coleorhiza ruptures due to radicle growth. After a while, the coleoptile emerges. Above the radicle, three seminal roots emerge (but variation in number). The radicle and seminal roots, along with two branches, remain throughout the plant’s life. The lowermost nodes above the mesocotyl form adventitious roots.
Germination of Wheat Grain
Hypogeal seed germination cycle of grain of wheat.
Wheat grain seed germination cycle details are similar to maize grain seed germination cycle details. Wheat grain germinates hypogeally.
Germination of Coconut
During germination, the embryo’s lower end forms the cotyledon, which grows as a spongy structure inside the endosperm. As it absorbs food material stored in the endosperm, this spongy cotyledon expands in size. The embryo’s upper end develops through the ‘eye,’ which contains the radicle and plumule.
Successive Stages of hypogeal seed germination cycle in seed of coconut.
The plumule emerges as a horn after piercing the fibrous pericarp. This causes the aerial shoot to grow even before the roots make contact with the soil. The radicle does not grow any further, but several adventitious roots sprout from the plumule’s base. Where the adventitious roots penetrate the soil, the seedling establishes itself.
The cotyledons are brought above the soil in seeds with epigeal seed germination cycle due to hypocotyl elongation. Flat green leaf-like cotyledons can be seen in young seedlings of castor, cotton, papaya, and onion. Apart from storing food, the cotyledons also perform photosynthesis until the seedling is self-sufficient. Other plants, such as beans, have thick cotyledons that do not develop into leaf-like structures; instead, they shrival and fall off after the seedling consumes their food reserves.
Germination of Gourd
Successive Stages of epigeal seed germination cycle in seed of Gourd.
The straight radicle emerges from the seed and anchors the seed to the soil, with secondary roots sprouting from the radicle. The hypocotyl then grows so quickly that it forms a loop that emerges from the soil and pulls out the rest of the seed. The seed coat is shed, and the cotyledons open out like two leaves, becoming green, large, and thin, and resembling ordinary leaves in appearance and behaviour. The plumule within the cotyledons is exposed and quickly develops into the aerial shoot.
Germination of Castor
Successive Stages of epigeal seed germination cycle in seed of castor.
The seed absorbs water, and the testa bursts near the caruncle, allowing the radicle to grow. As this hypocotyl grows, two papery cotyledons surrounded by endosperm are pulled from the soil. When the endosperm is consumed, cotyledons emerge. The cotyledons turn green and leafy, while the plumule gradually develops into a leafy shoot. Endosperm remnants wither and drop off.
Germination of Onion Seed
The radicle and the base of the scutellum (cotyledon) grow from the seed in this case. The radicle penetrates the soil, while the opposite end of the cotyledon remains within the endosperm and sucks the food material. The cotyledon base expands, turns green, and pushes the seed out of the soil. As long as the base of the cotyledon forms a sheath just above the radicle, the plumule is not visible.
Successive Stages of seed germination cycle in seed of onion.
The plumule has now pierced the cotyledonary sheath, forming the first cylindrical foliage leaf. Meanwhile, adventitious roots form a fibrous root system above the radicle (In this case the seed is pushed out of the soil by growth at the base to cotyledon and not by growth of hypocotyl).
The phenomenon of giving birth to young one at an advanced stage of development is known as vivipary. It is found in mammals (among other things) and mangrove plants. Because of the high salt content and lack of oxygen in marshy habitats, seeds of mangrove plants (e.g., Rhizophora, Sonneratia, Heritiera) cannot germinate on the ground. Seed dormancy is absent in such plants.
Vivapary A. Twig of Rhizophora showing viviparous germination. B. seeding has become established on tidal soil.
While the latter is attached to the parent plant, the embryo of the seed (present inside the fruit) continues to grow. The hypocotyl extends and pushes the radicle out of the seed and fruit. Growth continues until the hypocotyl and radicle reach a length of several centimetres (more than 70 cm in Rhizophora).
As a result, it loses contact with the fruit and falls into the salty muddy water in such a way that the plumule remains outside the salty water while the tip of the radicle becomes embedded in the mud. This safeguards the plumule. The radicle quickly establishes the seedling as a new plant by forming new roots.