1. Morphology of a Gram Seed (Cicer arietinum):
Gram seed (Fig. 8.2) is a dicot, non-endospermic seed. The seeds are produced within the pods or leguminous fruits.
A gram seed appears conical-pyriform in outline.
It has following parts:
(i) Seed Coat:
It consists of two layers-outer testa and inner tegmen. Testa is thick and brownish. The tegmen is thin, membranous, and whitish and remains fused with testa. The pointed beak like end of the seed has a minute pore called micropyle. If a soaked seed is gently pressed, a drop of water oozes out of the micropyle. A small oval scar seen near the micropyle is called hilum through which the seed was attached to fruit. Another oval scar present in the middle is called chalaza or strophiole. A distinct ridge called raphe runs from hilum to chalaza.
(ii) Embryo:
It presents inner to seed coat. It consists of two circular yellowish cotyledons that are attached to the embryo axis. The part of embryo axis above the point of attachment to the cotyledons is called epicotyle. The tip of epicotyle is called plumule. Similarly, the region of the embryo axis below the point of attachment of cotyledons is called the hypocotyle. The tip of hypocotyle is called radicle. During germination, the radicle comes out first through the micropyle and grows to form a tap root. The plumule gives rise to shoot system.
2. Morphology of Castor Seed (Ricinus communis):
Castor seed is a monocot, endospermic seed. The castor seeds are produced within a schizocarpic fruit called the regma which on maturity breaks up into 3 cocci, each containing a single seed. A castor seed is roughly oblong in outline with distinct convex (dorsal) and flat (ventral) surfaces. A castor seed has following parts (Fig. 8.3)
(i) Testa:
It is the outer layer of seed coat. It is thick, hard and brittle. The external surface appears smooth, shinning and mottled brown in colour.
(ii) Tegmen:
It is the inner layer of seed coat that appears dull and papery. Now it is called as perisperm or persistent nucellus.
(iii) Caruncle:
It is a white spongy bilobed outgrowth present near the narrow end of the seed. If partially covers the hilum (dark scar) and completely covers the micropyle (small pore). Caruncle absorbs water which percolates through the micropyle into the seed.
(iv) Raphae:
It is a shallow ridge present on the testa of flat surface of the seed. The distinct bifurcation of raphae represents chalaza.
(v) Endosperm:
It is a white oily food storage tissue that is present inner to the perisperm. From this layer castor oil of commerce is extracted.
(vi) Embryo:
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. Morphology of a maize seed (Zea mays):
Maize or Corn seed (Fig. 8.4) is actually a one seeded fruit called caryopsis or grain. It is a monocot endospermic seed.
It consists of following parts:
(i) Seed coat:
It is fused with the fruit wall (pericarp). It encloses a kernal which includes embryo and endosperm.
(ii) Endosperm:
It constitutes 2/3 of the grain. Endosperm consists of outer aleurone layer and inner starchy endosperm.
(iii) Embryo:
It lies on one side of the starchy endosperm and appears to be a lighter oval area in the whole seed. Embryo consists of a scutellum and a short embryo axis (tigellum). The scutellum is a shield-shaped cotyledon attached to a node of embryo axis. The surface of scutellum facing endosperm is called epithelial layer. It is both secretory and absorptive in nature. The epithelial layer secretes hormones into the endosperm for the synthesis of enzymes required for solubilisation of food. The solubilised food is absorbed by it and then transferred to the embryo axis.
The embryo axis has plumule (upper end) and radicle (lower end). The plumule contains a few rudimentary leaves and a conical protective sheath called coleoptile. The coleoptile has a termina 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 radicle has two protective sheaths, inner root cap and outer coleorhiza. 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.