The following points highlight the two main types of seed. The types are: 1. Dicotyledonous Seeds 2. Monocotyledonous Seeds.
Type # 1. Dicotyledonous Seeds:
i. Gram Seed:
The gram seed is more or less rounded at one end and pointed at the other. It is covered by a brown seed coat called testa; the inner whitish coat is the tegmen. At the pointed end of the seed the testa bears a scar called hilum.
It indicates the position of the attachment of the seed to the fruit wall. Near about the hilum there is a very minute opening called micropyle (micro=small; pyle = gate). If a soaked seed is pressed a droplet of water oozes out through this micropyle. The kernel is exposed when the seed coat is removed, which in the gram seed is nothing but the embryo.
If the kernel is pressed the two fleshy cotyledons are found, which remain laterally hinged to the axis of the embryo at a joint called the first node or nodal zone. The lower part of are found, which remain laterally hinged to the axis of the embryo at a joint called the first node or nodal zone.
The lower part of the axis lying towards the pointed end is the radicle and the upper part lying between the two cotyledons is the plumule. The plumule has very minute leaves and looks like a feather. Food matters remain stored up in the fleshy cotyledons. So the gram seed is dicotyledonous exalbuminous (Fig. 19).
Germination of Gram Seeds:
When the essential conditions are satisfied, the seed begins to germinate. At first it absorbs water and swells up. The testa ruptures near about the micropyle and the radicle is the first structure to come out of the seed.
It goes downwards to produce the main root (primary root) of the plant from which subsequently come out many branch roots. In the meantime the stalks of the cotyledons elongate to make way for the plumule to come out, which actually goes upwards being pushed by the active epicotyl.
The epicotyl is the region between the plumule and the point of attachment of the cotyledons, i.e. the nodal zone. The plumule gradually grows upwards to produce the green aerial portions of the plant. Food matters, stored up in the cotyledons, are used up by the growing embryo, so the cotyledons gradually shrivel up (Fig. 25).
It is to be noted that in the germination of gram seed the cotyledons remain inside the seed coat below the soil and the plumule goes upwards due to the active growth of the epicotyl. The position of the seed is not disturbed. This type of germination is called hypogeal or hypogenous (hypo=below; geo=earth). Peas and many other exalbuminous seeds show this type of germination (Fig. 26).
ii. Pea Seed:
Pea seed is more or less rounded in shape. It has only one coat, the pale white testa. On the testa there is the distinct hilum or the scar, and the micropyle is present near about the hilum. On removing the seed coat the kernel is exposed.
It is made up of two fleshy cotyledons attached laterally to the axis of the embryo, the lower part of which is the radicle and the upper part, the plumule. So the kernel in pea corresponds to the embryo. Pea is also dicotyledonous exalbuminous (Fig. 20).
iii. Castor-Oil Seed:
It is more or less oblong in shape. The testa is hard, shell-like and has many sculpturing’s on the outer surface. At one end the testa has a spongy outgrowth called the caruncle. The hilum and micropyle occur at that end and usually remain concealed by the outgrowth. Next to the hard brittle testa there is a thin white papery coat. It is not really the second coat or tegmen but is known as perisperm (remnants of nucellus).
The kernel consists of embryo and endosperm or albumen surrounding the embryo. The embryo has small radicle and very minute plumule forming the axis and two thin leafy cotyledons placed one against the other.
They have distinct veins which leave marking on the endosperm. Surrounding the embryo there is stored food matter, the endosperm. In a castor-oil seed the kernel is embryo plus albumen, and so it is a dicotyledonous albuminous seed (Fig. 21).
Dicotyledonous exalbuminous seeds ……. pea, gram, pulses, beans, mango
Dicotyledonous albuminous seeds …… castor-oil seed, papaw, poppy
Germination of Castor-Oil Seed:
In the castor-oil seed the seed swells up, the testa bursts near the spongy outgrowth, the caruncle, and the radicle comes out first. It goes downwards as usual to produce the primary root which later on bears many branches. Now the hypocotyl, the region between the radicle and the nodal zone, grows fast forming a curved arch or loop.
This loop gradually straightens up, thus lifting the plumule and the cotyledons in the air above the soil level. Even then plumule and cotyledons remain surrounded by endosperm and remnants of test. The endosperm is gradually exhausted. The two cotyledons develop green colour and serve as the first pair of green leaves. Now plumule grows upwards to form the green aerial shoot.
So in the castor-oil seed the plumule goes upwards by the active growth and elongation of the hypocotyl which pushes the plumule, cotyledons, etc., above the soil level. Thus the position of the seed is definitely disturbed (Fig. 27).
This is epigeal or epigeous germination (epi=upon, geo — earth). Gourd, tamarind also show this type of germination. In gourd the radicle usually develops, into the root and the Kypocotyl forms a loop which straightens up and pulls the rest above the surface of the soil.
A peg-like projection develops at the base of the hypocotyl which pushes the seed coat, so that the cotyledons and plumule may easily come out of the soil.
A peg like projection develops at the base of the hypocotyl which pushes the seed coat, so that the cotyledons and plumule may easily come out of the seed. The cotyledons become green and large and behave like ordinary leaves [Fig. 28(A)].
Tamarind seed also shows same type of germination [Fig. 28 (B)]. Here the testa is very hard and the two cotyledons are quite large and thick. They become greenish in colour but do not serve as green’ leaves. The cotyledons gradually shrivel up and drop when the stored food matters are exhausted.
Type # 2. Monocotyledonous Seeds:
i. Maize Grain:
The maize grain is really a one-seeded fruit and not a seed, hence it is called a grain. The seed coat and the fruit wall or pericarp are inseparably united in the maize grain to form the outer pale yellowish coat.
A whitish deltoid area is noticed on one side of the grain which marks the position of the embryo. The embryo can be clearly seen if the grain is cut into two parts along the longer axis. It occupies only a very small portion of the grain, the remaining part being endosperm or stored food.
The embryo has an axis with radicle and plumule. The radicle is surrounded by a sheath called radicle sheath or coleorhiza, and the plumule has a similar sheath called plumule sheath or coleoptile.
A shield-shaped body, called scutellum, lies between the axis of the embryo and the endosperm, thus dividing the grain into two unequal parts. Scutellum is the single cotyledon of the maize grain. It absorbs food from the endosperm for the embryo to be used during germination. Maize grain is monocotyledonous albuminous (Fig.22).
Germination of Maize Grain:
During germinations of the maize grain the radicle at first comes out boring through the coleorhiza or radicle sheath and forms the primary root. This root does not persist but dies and is replaced by a tuft of adventitious fibrous roots from the base of the stem. In the meantime the plumule goes upwards with the plumule sheath or coleoptile.
After growing to some extent the plumule pierces the sheath and grows up to produce the green aerial organs. Scutellum or the single cotyledon continues to supply food matters to the growing embryo during germination. The mode of germination in maize grain is also hypogeal (Fig. 29). Rice (Fig. 30) and other grains also have this type of germination.
In cocoanut the innermost layer of fruit wall is the shell having three eye-like scars, and the embryo lies below one of the scars. The white kernel including the milk is the endosperm. During germination the lower end of the embryo—the cotyledon begins to grow as a spongy structure to absorb food materials stored in the endosperm.
The upper end of the embryo grows through the eye carrying the spongy structure to absorb food materials stored in the endosperm. The upper end of the embryo grows the eye carrying the radicle and the plumule; pierces through the fibrous coat and ultimately establishes itself.
Special Type of Germination:
Plants growing in saline marshes or near sea-shore show a peculiar type of germination known as vivipary. In that case, the seeds begin germination before their liberation from the fruits (Fig. 31). The radicle becomes elongated and considerably swollen.
Then the seed gets detached from the parent plant and comes vertically downwards. The radicle pierces the muddy soil below and thus gets fixed. Lateral roots are soon formed for anchorage, and the plumule is kept above the surface of Valine water. Examples: Rhizophora, Ceriops.
ii. Rice Grain:
It is also a one-seeded fruit. The husk that encloses the grain is neither a part of the seed nor the fruit. It has four parts, two very small minute glumes, called empty glumes, at the base and two larger ones—the flowering glume and the palea, surrounding the grain.
The grain is exposed when the husk is removed. The reddish or whitish coat of the rice grain is the product of fusion of the pericarp and the seed coat. Here also the endosperm covers the major part of the grain, the minute embryo lying at one end beneath the palea.
During polishing of rice the outer coat and the embryo are removed, leaving a small break at one end of the grain. Obviously what remains after polishing is nothing but endosperm.
The embryo is more or less like that of maize having radicle and plumule with coleorhiza and coleoptile respectively and the shield-shaped scutellum, the single cotyledon. The radicle is a bit curved in rice grain (Fig. 23).
So it is also monocotyledonous albuminous. Most of the monocotyledonous grains are albuminous with a few exceptions like orchids.