The below mentioned article highlights the four important types of seed.
They are as follows: (1) Dicotyledonous Exalbuminous Seeds (2) Dicotyledonous Albuminous Seeds (3) Monocotyledonous Albuminous Seeds and (4) Monocotyledonous Exalbuminous Seeds.
Type # 1. Dicotyledonous Exalbuminous Seeds:
A typical example of this type is found in the common pea (Pisum sativum) . On carefully opening a mature green pod along the dorsal suture the placental tissue is seen to spread along the ventral suture and the roundish seeds are seen arranged in two rows along the length of the pod.
Each seed is attached to the placental tissue on the fruit suture by a stalk called the funicle. The funicle is narrow at the placental end but widens into a disc where it joins the seed. When the mature seed is detached the broad end of the funicle leaves a scar on the seed called the hilum. Next to the hilum is a pinhole opening on the seedcoat which is the micropyle.
If the seed is’ soaked, wiped and then squeezed, water is seen to ooze out of this micropyle. The seed is covered by the tough seedcoat of a light colour. This tough seedcoat is the testa.
The tegmen, which is delicate and completely adherent to the inner side of the testa, is not distinguishable in the mature seed. On opening the seedcoat the kernel is obtained. The kernel in dicotyledonous exalbuminous seeds is the embryo. In it the two fleshy cotyledons are very conspicuous. These arc fleshy as all the nutrients required by the growing seedling in the first few days are kept stored here.
The two cotyledons are hinged to an axis (tigellum) so that they open out like a book. The tigellum represents the axis of the future plant. One end of the tigellum is pointed and protrudes out of the cotyledons.
This lies next to the micropyle and is the radicle or the rudimentary root. The protruding radicle lies under the pouch-like expansion of the seedcoat and is thus visible even when the seedcoat is not removed.
The other end of the tigellum is the feathery plumule end which is the first apical bud of the future plant and develops into the shoot. The plumule lies in a groove inside the cotyledons. The point of attachment of the cotyledons to the tigellum is the first node on the axis and careful observation shows the presence of the first lateral buds in the axils of the cotyledons.
The portion of the tigellum just below the cotyledonary node (i.e., between radicle and node) is called the hypocotyl and the portion just above (i.e., between node and plumule) the node is the epicotyl.
All the dicotyledonous exalbuminous seeds conform to the above plan, though there may be /variations in details. In gram (Cicer arietinum), the seed is broad at one end and somewhat pointed at the other. On the seedcoat, below the hilum, there is another more prominent scar, the strophiole, which is a scar left by a funicular outgrowth.
The brown seedcoat is the testa but on its inner side a papery white membranous layer may be distinguished as the tegmen. The embryo does not differ from that of pea except in shape.
There are various types of bean seeds of which Dolichos lablab is very common. The seeds occur in the pod as in peas. They are larger and more or less oval. The seedcoat (mainly testa with a thin fused tegmen) is very hard and black, brown or red in colour. The funicle is extended into a long raphe which is seen above the hilum. The embryo is as usual.
There are quite a few other dicotyledonous exalbuminous seeds that we commonly come across. The tamarind (Tamarindus indica) seed is rectangular and covered by a very hard shell-like seedcoat.
The hard covering on the seed of mango (Mangifera indica) is actually the innermost layer of the fruit (endocarp) which is not a part of the seed. Inside, the two very large cotyledons are covered by a papery seedcoat. In jack-fruit (Artocarpus heterophyllus ) the two cotyledons arc of irregular and unequal size. The seedcoat shows both testa and tegmen. The cotyledons contain latex and, of course, lot of starch.
In the cucurbits (gourd, cucumber, etc., of Cucurbitaceae) the seed is rather flat. The testa is horny and free from the thin tegmen inside. The cotyledons also are flat although they contain a good amount of food matter including oil.
They show vein markings although white. The sunflower (Helianthus annuus) seed is actually its fruit enclosing a single seed. The hard covering is the fruit-wall or the pericarp.
Inside, the oily embryo is found covered by a brownish membranous seedcoat. Mustard, groundnut (peanut), sesame and flax (linseed) seeds contain lot of oil in the cotyledons.
In orange and lemon the seed is sometimes poly embryonic, i.e., it contains more than one embryo. The oak acorn is a fruit. The testa of the pomegranate (Punica granatum) is juicy and forms the edible part.
Type # 2. Dicotyledonous Albuminous Seeds:
In this type of seeds (ground plan) the food is not stored in the two cotyledons of the embryo but in the endosperm external to the embryo. It will be seen later that all embryos get their food from the endosperm which in its turn gets its food from the nucellus of the ovule.
In the exalbuminous type of seeds the embryo completely consumes the endosperm and nucellus so that they are no longer seen while the food is kept stored in the cotyledons which become swollen. In the albuminous type the endosperm is still present and the cotyledons are thin—acting only as food-sucking organs.
A very common example is the castor bean (Ricinus communis) where the fruit is not a bean but a three-chambered capsule. Here, the seedcoat is a hard shell of a mottled black or brown colour. The hilum is almost hidden by an outgrowth, the caruncle. The caruncle is spongy and absorbs water readily so that it may be of some use in germination: There is a distinct raphe running longitudinally down the seed from the hilum.
On breaking open the shell a white mass is found covered by a papery white membrane. This membrane is sometimes supposed to be the tegmen but has been found not to be a part of the seedcoat.
It is a remnant of the nucellus which has not been completely used up. Such a remnant is called the perisperm. Inside the membrane, the whole whitish, fleshy, slightly flattened and oval mass is the endosperm which contains much oil. On cutting open the endosperm the embryo is found to remain embedded inside. There are two thin, white cotyledons. The cotyledons show distinct vein markings like leaves. The veins leave an impression on the endosperm.
The two cotyledons are hinged to the tigellum which shows the protruding radicle with a short hypocotyl behind it and the plumule hidden between the cotyledons.
Among common plants, the dicotyledonous albuminous type of seed is also found in papaw (Carica papaya), jute, cotton, Mirabilis, etc.
Some whole families show this type of seed, e.g., Magnoliaceae, Annonaceae (custard apple shows a ruminated endosperm; other common plants are the mast tree or Polyalthia longifolia and Artabotrys hexapetalus), Papaveracear (the poppies).
The family Nymphaeaceae shows perisperm and aril (an Outgrowth like caruncle) in addition to the endosperm. In the water-lily (Nymphaea) of this family, the testa and tegmen surround a mass which is the perisperm.
Inside it, on the pointed end is the minute embryo embedded in a small endosperm. The raphe is prominent. The seed has a third covering called the aril which is spongy and helps the seed to float.
The family Piperaceae shows both endosperm and perisperm. The common black pepper (Piper nigrum) of this family is a fruit with a single seed. The shrivelled pericarp encloses a seed with a cellulose perisperm having an embedded endosperm and a tiny embryo. In coffee (Coffea arabica), the small embryo with two cotyledons is embedded in a mass of horny endosperm formed mostly of hemicellulose.
In the lime tree (Tilia europoea) the endosperm embeds an embryo in which the cotyledons are thin, palmately lobed and veined like leaves. Geranium molle shows a similar embryo within endosperm having cotyledons which are not only lobed but are also stalked.
Type # 3. Monocotyledonous Albuminous Seeds:
Most of the common monocotyledonous seeds are albuminous. The large endosperm of the cereals is the most important source of starch, the principal food of all people.
Rice (paddy), wheat and maize may be taken as the type seeds of this class. They are the most important cereal crops of the world. In all these, the- grains are actually fruits of the caryopsis type. The rice grain is tightly covered by the husks, in wheat the husks are loose, while in maize the husks are short and loose so that the grains are exposed. In all the three, the outer coating of the grain is formed by the fusion of the pericarp and the seedcoat.
The micropyle and the hilum cannot be found because of the pericarp covering. Inside, a large endosperm forms the bulk of the grain, while a small embryo occupies a comparatively small space on one side of the base. The outermost layer of the endosperm is the aleutone layer which contains mainly protein.
Of these three, maize (Zea mays) has the largest embryo and is the easiest to examine and dissect. Maize grains are flattened and more or less oblong. On the flat face the embryo can be seen even from the outside as a whitish deltoid area. The wall (pericarp + seedcoat) is yellowish or red and so firmly adherent that it cannot be separated from the kernel. If the seed be cut longitudinally into two and the, cut face stained with iodine, the endosperm part (deep blue because of starch) and the embryo part (yellowish) can be easily demarcated.
Usually, the upper and outer part of the endosperm is horny (more protein) while the inner part and the part nearest to the embryo is mealy white as it contains mainly starch. On the upper side of the embryo is a shield-shaped structure called the scutellum which completely covers the endosperm. The tissue of the scutellum abutting on .the endosperm forms the epithelium which is a glandular tissue secreting enzymes to digest the food in the endosperm.
The scutellum is actually the single cotyledon of monocots whose leaf nature is not at all clear. Its only apparent function is the absorption of food material from the endosperm.
Lateral to this scutellum is a short axis the lower part of which is the radicle covered by a sheath called the coleorkiza (root-sheath) and the upper part of it is the plumule showing a number of young leaves, sheathed by the coleoptile. A good preparation will show vascular strands emanating from the middle of the axis (the first or cotyledonary node) and ramifying in the scutellum. This is the path through which food travels to the axis. The region between the cotyledonary node and the base of the coleoptile (i.e., the first internode) is called the mesocotyl.
The wheat grain (Triticum ) essentially resembles maize. The grain is oval and there is a longitudinal groove along the ventral side. The wall is light brown (‘wheat colour’).
The endosperm contains a decreasing proportion of gluten (a protein) as it approaches the scutellum giving it a more and more mealy white appearance.
The embryo resembles maize but from the middle part of the axis, opposite the scutellum, a tongue-like outgrowth, called the epiblast, is seen. The epiblast is also found in many other Gramineae.
The rice grain (Oryza sativa) shows a brown or red-brown wall (pericarp + seedcoat). A lot of protein is contained in the aleurone layer which, along with the pericarp and seedcoat, gets rubbed off in course of polishing rice. The embryo also gets broken.
This impoverishes the polished grain in its vitamin, protein and oil contents. The epiblast is found here as well. One peculiarity of the rice embryo is that the plumule (which is of a later origin) and the radicle do not lie on one straight line but form an angle between them. In wheat and rice the embryo part is very small—only about 60% of the seed or less.
There is some controversy about the morphology of scutellum, coleoptile, coleorhiza, mesocotyl and epiblast. The scuteflum is usually regarded as the cotyledon but some botanists prefer to include the coleoptile and the mesocotyl within the cotyledon.
The coleorhiza also is regarded by some as a downward prolongation of the cotyledon. Others regard the mesocotyl as the first internode of the epicotyl and the coleoptile as a leaf. The epiblast is often regarded as a second suppressed cotyledon.
There are a few seeds of this type which are rather peculiar. In coconut (Cocos nucifera) the hard shell is the innermost layer of the fruit-wall (endocarp). There are three eye-like scars on the shell below one of which the embryo lies. On breaking open the shell, the seed is found covered by a dark-brown seedcoat which is adherent to the kernel. The white kernel, including the milk within it, is the endosperm. The embryo lies embedded on the ‘top’ (actually the base of the fruit) of the kernel below one scar.
In date palm (Phoenix sylvestris), palmyra palm (Borassus fiabellifer) and betel-nut (Areca catechu) seeds the arrangement is similar but the endosperm is more or less solid and horny containing reserve cellulose.
The stony seed of date is covered by a brown testa. The hard endosperm has a longitudinal groove on one side and on the centre of the other side the embryo is placed in a small pit.
The embryo is similarly placed in betel-nut and palmyra palm. In betel-nut the endosperm is ruminated as in Annonaceae.
The small black seed of onion is covered by a seedcoat. The inside is filled with a tough semitransparent endosperm within which a curved embryo is embedded. The embryo shows a radicle on one end and a scutellum on the other. The plumule is not seen at this stage. It develops later.
Many seeds belonging to the Scitamineae show the presence of both endosperm and perisperm. This is the case in cardamom (Ampmum) where-the endosperm and perisperm are oily and in Canna (Carina indica).
Type # 4. Monocotyledonous Exalbuminous Seeds:
Although all the common monocotyledonous seeds are albuminous there are a few of the exalbuminous type. This type of seed is found in the Aroideae (e.g., Pathos and Amorphophallus cam- panulatus) and also widely in the families Hydrocharitaceae (e.g., Vallisneria), Alismaceae (e.g., Alisma plantago), Naiadaceae, etc.
The seed of Alisma shows a curved embryo with a radicle and a cotyledon as shown in the figure.