Plant Reproduction: From Budding to Seed Formation
**Budding**
This type of multiplication occurs in some actively dividing cells, forming buds that are initially attached to the parent. In less developed plants, these buds are called propagules, and they eventually develop into independent new individuals.
**Fragmentation**
The parent spontaneously divides into two or more fragments, each of which forms a new individual. Types of fragmentation include:
- By rhizome: Horizontally elongated stems, often buried. Some of their buds give rise to new stems and roots, and therefore, to new plants.
- By trailing stems: These grow from the sides of the plant above the earth. When in contact with the ground, they develop roots and form a new plant.
- By tubers: Rhizomes thickened by the accumulation of reserve substances that become independent when the mother plant dies. They have buds that may develop into a new plant. These buds are called eyes.
**Sporulation**
This consists of the formation of spores, asexual cells able to originate a new individual without having to join another cell. They develop within plant organs called sporangia. This type of reproduction by spores in plants is only part of a cycle where sexual reproduction is also involved (diplohaplontic).
**Embryonic Period**
The zygote undergoes a series of successive mitoses to originate, first, two cells, one of which forms the embryo. The other cell gives rise to the suspensor, which binds to a cell of the embryo that is used to fix it later in the endosperm and will be the apex of the root. The embryo then begins to grow and differentiate. This entails the radicle, the hypocotyl (the part between the roots and the cotyledons), the epicotyl (above the cotyledons), and the gemmules. The endosperm, meanwhile, undergoes a series of mitotic divisions and synthesizes nutrients that will feed the embryo. In some plants, such substances are stored in the cotyledons, which increase in size, and the endosperm disappears. The primine and secundine of the egg become the seed coat, called the testa and tegmen. The fruit of angiosperms acquires a rigid consistency due to loss of water and is called a nut. In other cases, it stores nutrients and water, in which case it is called a fleshy fruit.
**Pollen Grain Formation**
The anther is divided into two compartments, each of which contains two microsporangia, where the pollen mother cells are located. Each mother cell undergoes meiosis, originating four microspores that are coated with a double wall: an outer, thick, and tough wall (exine) and an inner, thin, and delicate wall (intine). The nucleus of the microspore undergoes mitosis, resulting in a vegetative nucleus and another spermatic nucleus. This, in turn, is again divided by mitosis into two sperm nuclei. The result is the pollen grain with three haploid nuclei: the vegetative and the two spermatic.
**Female Reproductive System**
Each egg is protected by two integuments, the primine and secundine, that surround the surface almost entirely except for a small opening called the micropyle. Inside the egg is formed the nucellus, which is the true macrosporangium, and within it is the embryo sac mother cell. The stem cell of the embryo sac originates by meiosis four haploid daughter cells arranged in a row. Three of them degenerate, and the one closest to the micropyle becomes the embryo sac, which increases considerably in size, and whose nucleus undergoes mitosis three times repeatedly, resulting in eight haploid nuclei. Three of these cell nuclei are isolated as they are close to the micropyle and form the ovular apparatus, which consists of the oosphere in the central position and the synergids, on both sides of the oosphere. Another three nuclei, also isolated as cells, are located at the opposite end. This gives rise to the antipodes. The remaining two nuclei fuse and form a diploid nucleus, called the secondary nucleus, which is located in the center.
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**Floral Structure**
- The corolla has a protective function and attracts pollinating insects. It has colored leaves called petals. Together with the calyx, it forms the perianth.
- The androecium, or male reproductive system, is a cluster of stamens, each with a filament that widens at its terminal to form the anther.
- The calyx is composed of some leaves called sepals whose function is protective.
- The gynoecium, or female reproductive tract, is composed of two or more enclosed, bottle-shaped carpels that show a widened basal part, the ovary, which extends into a neck called the style, which opens to the stigma. The ovary contains one or more ovules inside, bound to it by a short stalk, the funiculus. The ovarian region where they attach is called the placenta.
- The floral receptacle is a widening of the stalk where the other pieces emerge: calyx, corolla, etc.