In this essay we will discuss about the process of germination of common seeds.
Essay # 1. Epigeal Type of Seeds:
a. Germination of an Exalbuminous Dicotyledonous Seed:
Gourd (Cucurbita Maxima of Cucurbitaceae):
Initially, after soaking water, the radicle comes out of the seed, goes straight downward and fixes the seed with the soil. The hypocotyl next grows very quickly and forms a loop-like structure and comes out of the soil. Gradually, it pulls the entire seed on the ground.
Similarly the lower half of the seed coat gets attached to the peg-like projection developed at the base of the hypocotyl. Due to this, the seed coats cast off very easily and the cotyledons come out in the air. The cotyledon then opens like leaves. Later on, the plumule (within the cotyledon) becomes exposed and develops into an aerial shoot (Fig. 2.8).
b. Germination of an Exalbuminous Monocotyledonous Seed:
Alisma Plantago of Alismataceae:
Initially, after soaking water, the hypocotyl comes out of the seed. From the base of the hypocotyl, a bunch of root hair develops around the insignificant radicle. After that, the cotyledon comes out of the seed as an erect stand by its own development. Meanwhile the radicle elongates to give rise to the primary root. The plumule develops into a shoot (Fig. 2.9).
c. Germination of an Albuminous Dicotyledonous Seed:
Caster (Ricinus Communis of Euphorbiaceae):
During germination, the hard testa first bursts near the caruncle. Gradually the radicle comes out and fixes into the soil. With subsequent growth of the hypocotyl, the cotyledons (enclosed by endosperm) come out of the soil and the testa cracks and cast off. The endosperm becomes consumed by the cotyledons.
The cotyledons open like leaves and become green in colour. The remnant of the endosperm gradually withers and drops off. The plumule between the cotyledons gradually grows up and develops into a new leafy shoot (Fig. 2.10).
d. Germination of an Albuminous Monocotyledonous Seed:
Onion (Allium Cepa of Liliaceae):
During germination, the radicle, along with the base of the curved cotyledon, comes out of the seed. The radicle penetrates the soil. The other end of the cotyledon remains in the endosperm and draws nutrition. The base of the cotyledon further grows and becomes green, which pushes the seed out of the soil.
For a long time, the plumule is not visible; as it covered by the cotyledonary base in the form of a sheath above the radicle. The plumule now pierces the sheath and forms the first foliage leaf. By this time, the adventitious root develops from the region above the radicle and forms fibrous root system (Fig. 2.11).
This type of germination is very rare in monocotyledonous plants.
Essay # 2. Hypogeal Type of Seeds:
a. Germination of an Exalbuminous Dicotyledonous Seeds:
i. Pea Seed (Pisum Sativum of Fabaceae):
In presence of all essential requirements at a satisfactory level, the radicle and plumule start to develop. The radicle usually grows first and comes out of the seed coat through the micropyle. It grows vertically downwards and, after entering the soil, produces first primary and then gradually secondary, tertiary branches in acropetal order — thus it forms a root system.
During the radicle development the epicotyl begin to grow, initially by forming a loop. Later on, it straightens and comes out of the cotyledons and seed coat. Then it grows rapidly and gives rise to stem and then leaves towards the apical region.
During the above process of germination, the cotyledon does not change its position and remains inside the seed coat. Hence the germination is of hypogeal type. The cotyledons are generally reduced in size and gradually wither away (Fig. 2.12).
ii. Gram Seed (Cicer Arietinum of Fabaceae):
The mode of germination of gram is similar to pea i.e., hypogeal type (Fig. 2.13).
b. Germination of an Albuminous Monotyledonous Seeds:
i. Paddy (Oryza Sativa of Poaceae):
After water soaking, the coleorhiza pierces the base of the grain and appears as a knob. The radicle then splits the coleorhiza and penetrates the soil. The coleoptile then comes out. Concomitantly, after the emergence of the radicle, two other roots grow from its base — the seminal roots.
The radicle and seminal roots give rise to secondary branches. Unlike dicotyledons, the radicle and seminal roots do not form the root system. At the base of the coleoptile, the mesocotyl becomes slightly elongated.
The plumule then comes out of the coleoptile. Adventitious roots then develop from the top of the mesocotyl i.e., from the base of hypocotyl. These adventitious roots actually form the fibrous root system. Within short time the coleoptile becomes damaged (Fig. 2.14).
ii. Wheat (Triticum Aestivum of Poaceae):
The germination of wheat is almost like paddy. But the number of seminal roots is generally 4-5. Adventitious roots also develop above the mesocotyl. Some of the adventitious roots may develop on the coleoptile.
The radicle and seminal roots probably persist along with the adventitious root and function normally (Fig. 2.15).
iii. Coconut (Cocos Nucifera of Arecaceae):
Initially, the small embryo remains undifferentiated below the eye on the endocarp inside the endosperm. During germination the lower end of the embryo develops a more or less round spongy structure — the cotyledon. It absorbs nutrition from the endosperm and becomes enlarged in size. The upper end develops through the eye carrying the radicle and plumule.
Later on, the plumule pierces the mesocarp and epicarp and finally comes out in the air in the form of a hornlike structure, which finally develops into an aerial shoot. The radicle does not develop any more. Several adventitious roots grow from the base of the plumule. The roots also penetrate the mesocarp and epicarp. Later on, the roots come in contact with the soil and establish the plant (Fig. 2.16) on the substratum.
Essay # 3. Vivipary or Viviparous Type of Seeds:
In this type, the seed — instead of being liberated from the fruit — germinates inside the fruit that is still attached to the mother plant.
Vivipary is common in some mangrove plants like Rhizophora mucronata, Ceriops roxburghiana, Kandelia rheedi etc., of Rhizophoraceae; Aegiceras majus of Myrsinaceae and Avicenia officinalis of Verbenaceae that grow in the Sundarbans in Gangetic delta and Indo-Burmese rivers. The embryo of this type of seeds has no resting period and continues to grow inside the fruit.
The radicle initially comes out of the fruit and then the hypocotyl begins to grow vigorously, thereby it looks like a club, whose length is usually 2-10 inch (5-25 cm). The hypocotyl points downward during its growth. Slight growth of the plumule also takes place, while the cotyledons remain inside the fruit acting as haustoria.
The Rhizophora (Fig. 2.17) grows in the deepest water and has largest hypocotyl. When the hypocotyl becomes heavy, the entire fruit or the elongated axis i.e., the radicle, hypocotyl and plumule get detached from the plant and fall vertically downwards into the mud present around the mother plant by its own weight and fixes itself in the ground.
It is however, visible in squash, Sechium edule of Cucurbitaceae; a vegetable fruit commonly found in Indian hill towns; where the seeds germinate inside the fruit still attached with the mother plant. It is also visible in coconut, Cocos nucifera of Palmae. Vivipary is also found in jack, orange, tomato, lemon, Agave etc.