Anatomy of dicotyledonous and monocotyledonous seeds, Unique features of plant development
Anatomy of dicotyledonous and monocotyledonous seeds:- The structure of monocot and dicot seeds can be described based on the following parts:
1. Seed Coat:-
> The seed coat is the outermost covering of the seed which in some cases might remain fused with the fruit wall.
> The seed coat is formed from two integuments or layers of cells present on the outside of the ovule. The tissue is derived from the mother plant where the inner layer forms the tegmen and the outer forms the testa.
> In some monocots, the layers of the seed coat are not distinct and might even remain fused with the fruit wall. The outer layer, when distinct, consists of patterns or some patches of hair.
> The number of layers of the seed coat depends on the characteristic of the ovules. In the case of bitegmic ovules, the inner layer either remains as a single layer or divides to form two or three layers and accumulate food materials.
> The outer layer, in turn, contains cells with tannin deposits, resulting in a dark-colored appearance.
> As the cells of the seed coat begin to enlarge, the outer layer continues to deposit different substances that cause their walls to thicken.
> The seed coat contains an oval depression called hilum which represents the point where the ovules were attached to the wall of the ovary.
> The seed coat of some seeds might have hair or wings which help in the dispersal of the seed by the wind. Similarly, some seed coats are composed of waterproof materials to protect it from drying or decaying during dispersal by water.
2. Endosperm:-
> The endosperm is a mass of tissues formed within the seed during the process of fertilization.
> The cells of endosperm are unique in that they are triploid with three sets of chromosomes per nucleus.
> The primary purpose of endosperm is to surround the embryo and provide nutrition to the embryo.
> The formation of endospore requires one of the sperm cells to fertilize with the diploid central cell of the female gametophyte. This results in the formation of primary endosperm cell with triple fusion nucleus.
> Most of the flowering plants are polyploidy, but others might have a triploid or diploid set of chromosomes.
> The size of the endosperm is quite big in monocots as endosperm is the primary source of nutrition for the embryo. In dicots, however, the nutrient is provided by the two cotyledons.
> The endosperm consists of three different types of cells; the starchy endosperm cells, the basal transfer layer, and the aleurone layer.
> Most of the endosperm is occupied by the starchy endosperm. The endosperm is composed of dead cells filled with starch granules and protein bodies.
> The cells of the basal layer are characterized by the presence of cell wall ingrowths with cell membrane that is up to 22-fold more than normal plant cell.
> The aleurone layer is a single-cell layer, but in some monocots, the layer can be three-layered thick. The layer surrounds the starchy endosperm and the embryo.
3. Embryo:-
> Embryos are the simple multicellular structure of undifferentiated cells formed as a result of fertilization of haploid egg cell by a sperm cell.
> The embryo is composed of DNA obtained from the ovule as well as pollen forming a zygote.
> The embryo is the part of a seed and is protected within the seed by various structures like endosperm and seed coat.
> The zygote resulting from fertilization undergoes first cell division which is asymmetrical. The asymmetrical division results in the formation of an embryo with a small cell and a large cell.
> The small apical cells eventually develop to form parts like stem, leaves, and roots, whereas the larger cell gives rise to the connective. The connective connects the embryo to the endospore.
> As the embryo continues to grow, it separates into distinct regions in which cell division occurs and areas where non-reproductive activities like metabolism, respiration, and storage take place.
> The embryo has the highest concentration of lipid and lipid-soluble vitamins among all the other parts of the seed.
> In monocots, the embryo is present in a groove at one end of the endosperm with one larger shield-shaped cotyledon called scutellum.
> The embryo axis contains other structures like plumule, radicle, hypocotyl, and epicotyl that eventually lead to the formation of a new plant by embryogenesis.
4. Plumule:-
> The plumule is a part of the seed embryo that eventually develops into the shoot with vegetative parts like leaves and stems.
> It appears as a bud at one of the axis of the embryo and is often termed as the embryonic shoot.
> Plumule is negatively geotropic unlike radical and might or might not contain leaf structure when present within the seed.
> In some plants like sunflower, the plumule has no leaf structure, and the growth also doesn’t occur until the cotyledons are present above the ground.
> In others, however, a leafy structure is present which develops through the soil with cotyledons still present below the surface.
> In monocots, the plumule is surrounded by a coleoptile which is absent in the plumules of dicot seeds.
5. Epicotyl:-
> Epicotyl is the region of the embryo that is present above the stalks of the seed leaves, which is essential for the beginning stages of plant germination.
> Epicotyl grows rapidly than other parts of the embryo and exhibits hypogeal germination. Hypogeal germination is defined by the growth of plumule above the soil while the cotyledons are still below the surface.
> The growth of the epicotyl region is responsible for the extension of the stem above the soil surface.
> The growth of the cells of the epicotyl region forms the point of attachment between the shoot apex and the first true leaves of the embryo.
> The concept of epicotyl is different in monocots and dicots. In monocots, the shoot that emerges from the soil or the seed is considered the epicotyl whereas, in dicots, the epicotyl represents the region of the shoot above the cotyledons.
6. Hypocotyl:-
> Hypocotyls is the region of the seed present below the seed leaves (or cotyledons) and bove the radicle.
> Hypocotyl eventually becomes part of the stem, but one of its essential functions is to push the cotyledons outer of the soil surface during germination.
> Even though hypocotyl doesn’t grow as rapidly as the epicotyl, it is the first structure of the plant that emerges out of the soil.
> The hypocotyl, along with the radical, makes way for the epicotyl as the cells of the epicotyl are delicate and might suffer damage during growth.
7. Radicle:-
> Radicle is the region of the embryo, which is the first to emerge from the seed during germination that eventually leads to the formation of roots.
> Radicle is often called the embryonic root as it is positively geotropic and grows downwards into the soil. The radicle comes out of the seed through the micropyle.
> Radicels are of two types depending on their orientation; antitropous radicles point away from the hilum on the seed coat whereas syntropous radicles point towards the hilum.
> The radicle in monocots is surrounded by a thin sheath called coleorhiza, whereas that in dicots, is not. The radicle does, however, have a root cap that protects it during the early growth stages.
8. Cotyledon:-
> Cotyledon is the embryonic leaf in the seed which occupies most of the space in the seed and provides nutrients and protection to the embryo.
> The number of cotyledons present within a seed is one of the characteristics used to differentiate flowering plants into different groups.
> Seeds with two cotyledons are termed dicotyledonous whereas seeds with a single cotyledon are termed monocotyledonous.
> In dicot plants, the cotyledons are photosynthetic and functionally similar to plant leaves.
> In monocot plants, the cotyledon is modified to form a scutellum which is non-photosynthetic and functions to absorb stored food from the adjacent endosperm.
> The number of cotyledons can be higher than two as well, like in pines and cypress that have about 24 cotyledons.
> The life of a cotyledon differs in different species as in some it lasts for a few days whereas in others it lasts for up to a year.
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Unique features of plant development:-
1. Rigid Cellulose Walls:- No cell and tissue migration. The three basic tissue systems (dermal, ground and vascular) formed without gastrulation (animals) to establish this layered system of tissues. Plant cells divide and differentiate in place.
2. Germ cells are not set - aside early in development. This is also the case in several animal phyla, but it is always true for plants.
3. Plants have extended embryogenesis. Meristems persist long after embryogenesis.
4. Plasticity in development:- One cell can regenerate whole plant. Compensates for the plant's lack of mobility.