In this article we will discuss about the meaning and types of seeds.
Meaning of Seeds:
Seed is the name of a ripened ovule which contains an embryo or miniature plant in suspended animation, adequate reserve food for future development of the embryo and a covering for protection against mechanical injury, loss of water, pathogens, etc. A seed may have one or two coverings called seed coats. The outer or the only seed coat (if one is present) is called testa while the inner one is named as tegmen.
The surface of the seed possesses a fine pore at one end. It is called micro Pyle. There is also a scar called hilum. It is the place where funiculus or stalk of the seed is borne.
Some seeds also show chalaza (place of origin of seed coats) and raphe (part of funiculus fused with seed wall). The embryo consists of an axis or tigellum to which are attached one (in monocotyledonous seeds) or two (in dicotyledonous seeds) seed leaves or cotyledons.
The place of attachment of cotyledons on the embryo axis is called cotyledonary node. Micro pyler end of tigellum bears radicle or embryonic root. The other end contains plumule or embryonic bud. The part of embryo axis or tigellum between the radicle and cotyledonary node is known as hypocotyl while the one between plumule and cotyledonary node is called epicotyl.
Food storing tissue of a seed is endosperm. In flowering plants it is produced as a result of double fertilization. In most monocots and some dicot seeds, the food reserve remains in the endosperm. They are called endospermic or albuminous seeds, e.g., cereals, caster Bean, Coconuts, Rubber.
However, the majority of dicot seeds (e.g., Pea, Gram, Bean, Mustard, and Groundnut) and a some monocot seeds (e.g., Orchids, Sagittaria), the endosperm is consumed during seed development and the food is stored in cotyledons and other regions.
They are called non-endospermic or exalbuminous seeds. In Nymphaea, the food is stored in nucellus or perisperm. The seed is called perispermic seed. In Bartholettia (Brazil nut), hypocotyl is enlarged to store reserve food.
Types of Seeds:
1. Gram Seed (Cicer arietimum):
It is formed in a small pod or legume.
External Structure (Fig. 5.134 A):
The gram seed is light or dark brown in colour. Its surface may be smooth or wrinkled. The seed is conicalpyriform in outline. It is pointed like a beak at one end.
The broad end bears a small notch or furrow which extends towards the pointed end on one side of the seed. The furrow bears in the middle an oval or rhomboidal dark patch called chalaza (also called strophiole). The same side bears a small oval scar in a depression towards the pointed end.
The scar is known as hilum. It is the point where the stalk or funicle of the seed is attached to it. A narrow ridge called raphe runs from hilum to chalaza inside the furrow.
In between the hilum and the pointed end is found a small pore. It is termed as micro Pyle. Water enters the seed through this pore. This can be observed by pressing a soaked seed when a drop of water or air is found to ooze out of it.
Internal Structure (Fig. 5.134 C):
The seed is covered over by two seed coats. The outer seed coat is known as testa. The inner seed coat is termed as tegmen. The testa is leathery, that is, thick and tough. The colour of seed is due to it. The tegmen is thin, delicate, transparent and papery. It lies closely in contact with the testa.
The seed coats enclose the embryo. The embryo contains a short and straight embryo axis (Fig. 5.134 D). Two large and broad yellow cotyledons or seed leaves are attached to the embryo axis by narrow and short stalks. The embryo axis is actually present pressed between the two cotyledons.
The tip of embryo axis towards the micropylar end is called radicle. It is the future root. The other end of the embryo axis found deeper in between the cotyledons is the plumule or future shoot.
It bears small or rudimentary leaves. The area of embryo axis between the point of attachment of the cotyledons and the plumule is described as epicotyls while the region between radicle and the place of origin of cotyle.
Gram seed is dicotyledonous. The stored food is also present in the cotyledons. An endosperm is absent. Gram seed is, therefore, non-endospermic.
2. Castor Oil Seed (Ricinus communis, vern. Arind):
The seeds are produced in a spiny regma which later breaks up into three cocci, each possessing a single seed.
External Structure (Fig. 5.135):
Castor oil seed is roughly oblong in outline. It is narrow at one end and broad at the other. The seed has two surfaces, ventral and dorsal. The ventral surface is flat while the dorsal surface is convex. The seed is mottled dark brown in colour with a smooth and shining surface.
The narrow end of the seed bears a white spongy and bilobed structure called caruncle. The caruncle is an outgrowth of the micropylar region of the seed. It helps the seed in the absorption of water.
At the base of the caruncle on the ventral side and partly concealed by it, is found a dark scar. It is spoken as hilum. The hilum is the place at which the funicle or stalk is attached to the seed. Near the hilum and completely covered by caruncle is found micropyle.
A shallow ridge, called raphe, runs along the middle of the flat surface of the seed. It gets divided into two near the broader end. The place of division of the raphe represents chalaza.
Internal Structure (Fig. 5.135 C-E):
The seed is covered on the outside by a thick hard but brittle shell called testa. The outer surface of the testa is shining and mottled dark brown while its inner surface is dull and light brown in colour.
Castor-oil seed is roughly oblong in outline. It is narrow at one end and broad at the other. The seed has two surfaces, ventral and dorsal. The ventral surface is flat while the dorsal surface is convex. The seed is mottled dark brown in colour with a smooth and shining surface.
Inner to testa is a delicate white and papery structure called perisperm. It was previously regarded as inner seed coat or tegmen. Perisperm represents the persistent nucellus. It encloses a white oily mass called endosperm. Endosperm stores reserve food of the seed. The reserve food is in the form of oil drops and proteins. Castor-oil of commerce is got from it after removing some harmful ingredients.
Embryo lies in the centre of endosperm. It consists of a radicle, a plumule and two lateral cotyledons, all of which are present on a short embryo axis. The cotyledons are thin, semi-transparent and oval in outline. They have palmate venation. The middle costa or rib is more prominent and bears a few lateral veins.
Radicle lies outside the cotyledons towards the micropylar end. It is a knob-like outgrowth. Plumule lies in between the two cotyledons and is quite indistinct. Epicotyl is also indistinct. In between the place of origin of the two cotyledons and the radicle is present a short hypocotyl.
Castor oil seed is dicotyledonous (having two cotyledons), endospermic (with a special food storing tissue called endosperm) and perispermic (having perisperm or persistent nucellus).
3. Maize Grain (Fig. 5.136):
Maize grain is a single-seeded fruit (caryopsis) where the fruit wall (pericarp) and the seed coat (testa) are inseparably fused.
The grains are produced on a large and swollen peduncle called cob:
External Structure (Fig. 5.136 A):
Maize grain may be whitish, yellow, violet or red in colour. It has a smooth and shining surface. The grain is conical and flattened. It is attached to the cob by its narrow pointed end which is surrounded loosely by a shallow husk.
The broader end is roundish. Near the broader end the upper flat surface contains a small papilla which represents the remains of the style. The same side possesses a deltoid depressed whitish area. It has a central ridge which indicates the position of underlying embryo axis.
Internal Structure (Fig. 5.136 B):
On the outside of the grain is present a single thin but hard covering. It is formed by the fusion of the seed coat or testa and the fruit wall or pericarp.
Below the grain covering are present two structures, endosperm and embryo. The endosperm occupies most of the interior of the grain on the broader and the lower sides. It consists of two parts, horny aleurone and mealy storage.
The aleurone region lies immediately below the grain covering. It is 1-3 celled in thickness. The cells have thick walls and dense cytoplasm filled with aleurone or protein or protein grains. The latter produce enzymes during the process of grain germination.
The storage region of endosperm is whitish or yellowish. It has large thin walled cells with disintegrated cytoplasm and rich in starch grains. The cells also possess fats and proteins. The embryo occurs in the pointed part of the grain, mostly towards the upper side.
It consists of an embryo axis containing a radicle, a plumule and a single lateral cotyledon. The radicle (or future first root) lies at pointed end of the grain. It has two protective sheaths, inner root cap and outer coleorhiza. The plumule (or future shoot) lies towards the broader side of the grain at the other end of embryo axis.
It bears a few rudimentary leaves and a conical protective sheath known as coleoptile. Coleoptile has a terminal pore for the emergence of first leaf during germination. The sheath is capable of growth. It assists the future shoot in passing through the soil during germination.
The single cotyledon of Maize grain is called scutellum. It occupies the major portion of the embryo region of the grain. The outermost layer of scutellum lying at the boundary of endosperm and embryo is known as epithelial layer. It is both secretory and absorptive.
The epithelial layer secretes hormones into the endosperm for the synthesis of enzymes required for solubilisation of food. The solubilized food is absorbed by it and then transferred to the embryo axis. Roughly in the middle of embryo axis arises a vascular strand. It ramifies into the scutellum. The place of origin of the vascular strand from the embryo axis is called cotyledonary node.