|Angiosperm Plant Formation|
Angiosperms are flowering plants. Their formation entails development from embryo to seed, through germination to seedling, and finally to mature plant.
The life cycle of angiosperms (flowering plants) involves an alternation of generations between a dominant sporophytic (spore-producing) phase and a reduced gametophytic (gamete-producing) phase. The first cell of the sporophyte is the fertilized egg, or zygote, which undergoes repeated divisions, growth, and differentiation to form an embryo enclosed in the ovule.
After fertilization, the ovule is transformed into the seed, which germinates into a seedling. The seedling becomes the adult plant; the plant produces flowers inwhich the sperm and egg—representing, respectively, the male and female gametophytic generations—are formed. Fertilization occurs, and seeds are produced to continue the life cycle.
Dicot Embryo Formation
In most angiosperms, embryo development, or embryogenesis, is initiated with a division of the fertilized egg into a small apical cell and a large basal cell, forming a two-celled proembryo. The apical cell generates the embryo proper, and the basal cell forms a filamentous suspensor that anchors the embryo.
Two weeds, Capsella bursa-pastoris (shepherd’s purse) and Arabidopsis thaliana (mouse ear cress or wall cress), both belonging to the Brassicaceae family, have attained prominence as textbook examples of embryogenesis in typical dicots (plants with two cotyledons, or seed leaves; a monocot has one seed leaf).
In these plants, the apical cell of the proembryo divides by two successive longitudinal walls to forma quadrant that is immediately partitioned by transverse walls into an octant, composed of an upper and lower tier of four cells each.
The fates of the two tiers are already fixed in the octant embryo, as the upper tier forms the shoot apex and much of the cotyledons. The lower tier, in addition to providing derivatives to the remaining part of cotyledons, generates the hypocotyl, the radicle, and the root apex.
However, the central region of the root cap, known as the columella, and the quiescent center of the root apical meristem are derived from the terminal cell of the suspensor closest to the embryo, known as the hypophysis. The apicobasal pattern of the future seedling plant is established in the octant embryo.
A series of divisions separating eight peripheral cells from a core of eight inner cells heralds histogenesis in the embryo. The result is the formation of a sixteen-celled, globular embryo, in which the peripheral cells form the protoderm (precursor cells of the embryonic epidermis), and the inner cells differentiate into the procambium and ground meristem (precursors of the vascular tissues and ground tissues, respectively) of the mature embryo. This initiates the formation of radial-pattern elements made up of concentric tissue layers in the basal part of the embryo.
|Dicot Embryo Formation|
Emerging from the globular stage, the embryo expands laterally by cell divisions to formthe cotyledons and becomes heart-shaped. The heart-shaped stage is followed by the torpedo-shaped stage, in which elongation of the cotyledons and hypocotyl, as well as extension of the vascular tissues, occurs.
The basic body plan of a shoot-root axis becomes unmistakably clear at this stage, with the establishment of the shoot apical meristem in the depression between the cotyledons and the organization of a root apex by incorporation of derivatives of the hypophysis at the opposite end of the embryo.
During further growth, the cotyledons bend toward the hypocotyl (bent cotyledon or walking-stick shaped stage), and the embryo is phased into the mature stage. A mature embryo of Arabidopsis has fifteen thousand to twenty thousand cells and, under favorable conditions of growth, develops in about nine days fromthe time of fertilization to the mature embryo stage.
Sensitive genetic screens have led to the isolation of Arabidopsis mutants defective in apicobasal and radial patterning of embryos. Characterization of the mutant genes and their protein products has unraveled to some extent the molecular components of the embryonic pattern-forming system in this plant.
Monocot Embryo Formation
The early divisions of the zygote in monocots follow the same pattern as in dicots. However, in the Poaceae (grasses) family, which includes wheat and the other cereals, the sequence and orientation of later divisions in the proembryo are irregular, resulting in highly complex mature embryos. The main feature of the cereal embryo is the development of an absorptive organ known as the scutellum (considered equivalent to the single cotyledon).
Other organs of the embryo for which there are no counterparts in the dicot embryo are a sheath like tissue covering the root (coleorhiza), a tissue that covers the shoot (coleoptile), and an internode known as the mesocotyl. On one side of the coleorhiza there is also a small, flaplike out growth called the epiblast.
Embryo Maturation to Seed
|Embryo Maturation to Seed|
In other plants, especially the cereals, the bulk of the seed (grain) is made up of the endosperm surrounding the small embryo. The mature embryo enclosed in the seed consists of an axis bearing the radicle (embryonic root) at one end and the plumule (the embryonic shoot consisting of the shoot apex and one or two leaves) at the other end, and one (in monocots) or two (in dicots) cotyledons.
The part of the embryo axis above the point of attachment of the cotyledon(s) is known as the epicotyl, whereas the part below the attachment point connecting to the radicle is called the hypocotyl.
The dry seed enclosing the mature embryo may not germinate immediately; if it does not, it enters a period of quiescence or dormancy. Quiescent seeds germinate when provided with the appropriate conditions for growth, such as water, a favorable temperature, and the normal composition of the atmosphere.
Dormant seeds germinate only when some additional hormonal, environmental, metabolic, or physical conditions are met. In almost all seeds, the first part of the embryo to emerge during germination is the radicle. It forces it way through the soil and forms the primary root of the seedling. However, the manner in which the shoot emerges and develops varies considerably in different seeds.
In the epigeous type of germination (for example, in beans), emergence of the radicle is followed by the elongation of the hypocotyl, which arches above the soil surface as a hook. As the hook straightens, it pulls out the cotyledons and plumule above the soil surface. In the hypogeous type of germination (in peas, for example) the cotyledons enclosed within the seed coat remain in the soil during germination.
It is the epicotyl that arches above the soil surface, and as the hook straightens out, it carries the plumule along with it to the surface of the soil. In the monocot, such as the onion, after emergence of the radicle the single cotyledon arches above-ground and subsequently straightens.
Members of the Poaceae display a type of germination in which, following the outgrowth of the radicle, the coleoptile enclosing the plumule grows out of the grain and appears above the soil. The seedling leaves force theirway, breaking the coleoptile, and appear outside as the first photosynthetic organs.
The growth of the coleoptile during germination of grains is facilitated by the elongation of the mesocotyl. These various types of germination ensure an efficient use of food materials stored in the embryo or in the endosperm for the growth of the seedling until it becomes autotrophic.
Embryo to Adult Plant
Although the question as to whether the seedling will become a gigantic tree or a small, herbaceous plant is determined by its genetic blueprint, certain common postgermination growth and developmental episodesmark the development of the seedling into an adult plant.
In dicots, continued growth of the primary root produces an extensively branched root system consisting of secondary roots or lateral roots. In monocots, the primary root disintegrates shortly after it is formed, and so the root system is constituted of numerous adventitious roots which arise from the base of the stem.
Although the cotyledons retain their photosynthetic capacity for some time after germination of the seed, the seedling becomes completely autotrophic as the shoot apex produces new leaves and branches arise in the axils of leaves.
These activities are coordinated by the division of cells in the root and shoot apical meristems and the differentiation of cells into specialized tissues and organs. The shoot and root apical meristems, considered analogous to the stem cells of animals, remain active throughout the life of the plant and, hence, are known as indeterminate meristems.