Flower Morphology and Anatomy: A Comprehensive Guide
Flower Morphology and Anatomy
Introduction
Flower: A specialized stem or bud where internodes are reduced and the leaves are modified into antophylls. The modification is more profound from the outside to the inside.
Parts of a Flower
Stem: The flower stem, a small stalk with a widened tip called the receptacle or thalamus.
Flower Wraps:
- Perigone (tepals or calyx)
- Perianth (calyx and corolla)
Essential Bodies:
- Androecium (stamens)
- Gynoecium (carpels)
Anatomical Structures
Peduncle: The epidermis, ground tissue, and vascular tissue.
Floral Wraps: Tepals, Sepals, and Petals:
Aestivation can be valvate, contorted (regular), or imbricate (irregular). Coloration is due to pigments such as carotenoids, which are in plastids, and flavonoids, which are deposited in vacuoles. Anthocyanins are among the 16 flavonoids, including cyanidin (magenta), delphinidin (purple), and pelargonidin (red), associated with alkaloids. Betalains, flavones, flavonols, and aurones are also present. The epidermis, mesophyll, and vascular system are also present.
Essential Bodies: Androecium
Stamens: The set of stamens represents the male organ of flowers. It occupies the position external to the ovary.
Parts of the Stamen:
Filament: It has an amphicribral, perifloematic vascular bundle.
Anther: Made up of two nests or lobules.
Wall Formation of the Anther: There are four types of wall formation, based on the behavior of the two secondary parietal layers produced by the first periclinal division of the first wall layer.
- Basic Type: The two secondary parietal layers divide periclinally and form a wall of four layers: endothecium, two middle layers, and the tapetum. This type is found in Tilia and is assumed to be derived from other dicots.
- Dicot Type: The outer layer of the two parietal layers divides to form the endothecium and the middle layers, while the inner layer remains undivided and forms the tapetum directly. This is the most common type and is found in numerous families. The sporangium wall consists of three layers.
- Monocotyledoneae Type: The inner secondary parietal layer divides periclinally, giving rise to the tapetum and the middle layer. The outer secondary parietal layer forms the mature endothecium. This type has three layers and is characteristic of Liliopsida.
- Reduced Type: Neither of the secondary parietal layers divides; they mature and form the endothecium and the tapetum directly. There is no middle layer.
Wall Formation of the Anther or Microsporangia:
- Epidermis: The outer layer may remain intact, sometimes compressed or released during anther maturation. In some species, the epidermis has trichomes; in others, it develops fibrous bands similar to the endothecium, called the exothecium.
- Endothecium: The inner layer of the epidermis, or the outer layer if the epidermis is absent, also called the fibrous layer. It has thickenings in the form of bars or networks on the anticlinal and tangential walls of the cells. In primitive families, it may be absent. This layer is associated with anther dehiscence and does not develop thickenings in the stomium area. The endothecium contains lignin and suberin.
- Middle Layers: Composed of one or more layers of tubular cells that are decompressed or destroyed during microspore formation.
- Tapetum: The inner layer of the microsporangium wall, with three functions: nutrition of the microspores, formation of the exine, and synthesis of materials that are part of the pollenkit and tryphine deposition. In angiosperms, the tapetum can be glandular (secretory), in which case the cells remain intact and persistent in situ, or periplasmodial (amoeboid), where there is rupture of the walls and fusion of the protoplasts. The protoplasmic mass forms a multinucleated periplasmodium that mixes with the pollen mother cells. The periplasmodial tapetum is usually uniseriate or multiseriate. The tapetum is also involved in activities such as maintaining a delicate balance with the differentiation of the sporogenous tissue, and any alteration in it can lead to pollen degeneration. Due to physical and physiological factors, the tapetum can lead to abortion of pollen, the endothecium, or the vascular bundle of the filament. The tapetum can also induce abortion of pollen, endothecium, or vascular bundle.
Ubisch bodies formed in the tapetal cells have carotenoids and carotenoid esters, which are sites of sporopollenin polymerization. The pollenkit is a compound composed of lipids and carotenoids, synthesized in the tapetum. It provides adhesiveness to the pollen wall, attracts pollinators, protects pollen from ultraviolet radiation, and adheres to the body of insects to facilitate dispersion. In entomophilous flowers, the pollenkit is homogeneous, while in anemophilous flowers, it is heterogeneous.
After meiosis, the microspores mature, and the tapetum dehydrates as tryphine is deposited on the surface of the pollen grains.
Anther Dehiscence
The release of pollen grains is carried out through the stomium. It can be of several types: poricidal, longitudinal, transverse, valvular, or longitudinal fissure (e.g., Orchidaceae, Araceae, Asclepiadaceae).
Structure and Method of Pollen
In most plants, pollen grains are free cells (monads). In some families, there are composite grains (tetrads), which may be 8, 16, or 32 pollen grains or complex masses (pollinia). In tetrads, the fertile grains may have 0, 1, 2, or 3 apertures. The shape of the pollen grains is a determined character, genetically set in three ways: by the position of the microspores, by the number and arrangement of germinal apertures, and by differential cell expansion during microspore development.
Formation of Pollen Grains
The life of the microspore ends with the first mitotic division, which gives rise to the vegetative and generative cells.
Structure of the Pollen Grain Wall
It is composed of two layers: the exine (outer layer incorporating sporopollenin) and the intine (inner layer, smooth, composed of polysaccharides).
There are two types of exine:
- Tectate: Has a surface layer composed of arches supported by a tectum, columellae, and a foot layer.
- Atectate: Where the structure of the surface layer is open.
Layers a and b are called ektexine, while layer d is called endexine. In Asteraceae, the two layers are separated by spaces called nexine or cavea.
Openings of Pollen Grains
Pollen grains can be inaperturate (no openings) or omniaperturate. Most have areas without exine or covered by a layer or operculum where the pollen tube emerges. Openings can be pores (porate) or furrows (colpate if the furrows are parallel to the equatorial plane, or sulcate if the furrows are perpendicular to the equator).
Gynoecium
The gynoecium consists of carpels or megasporophylls and is the female organ of the flower. It occupies the center of the flower.
Parts of the Gynoecium
Ovary: The place where the ovule or ovules are located. It has the following parts:
- Wall: Has stomata, hairs, cuticle, epidermis, parenchyma, sclerenchyma, and idioblasts with crystals and tannins.
- Locules: Spaces where the ovules are located. There can be one or several locules.
- Septa: In multilocular ovaries, also called partitions.
- Ovule: It has ground tissue, parenchyma, and vascular tissue that crosses it longitudinally. Placentation is related to the location of the ovule and can be: suspended, pseudobasal, parietal, axial, or central free. When the ovule is not directly associated with the ovary wall, it is called stalked; if there is a connection, it is called sessile.
Ovule or Egg: Composed of the integument (the outer primary and inner secondary integuments). Not all ovules have two integuments; they can be bitegmic, unitegmic, or ategmic (formed by suppression or fusion). The integuments have an opening called the micropyle. There is a layer called the endothelium that surrounds the embryo sac.
Nucellus: Nutritive tissue that can be crassinucellate (massive) or tenuinucellate (large or small).
Chalaza: The region between the raphe and the nucellus where the integuments are initiated.
Funiculus: Connects the ovule to the chalaza and ovary wall.
Raphe: The part of the funiculus that extends parallel to the nucellus.
Embryo Sac: Contains the egg apparatus, a filiform apparatus located at the micropylar pole, where there are three cells: a large oosphere and two synergids. At the chalazal pole, there are three antipodal cells.