Plant Reproduction and Structure: From Ovary to Seed

Plant Reproduction: From Ovary to Seed

Carpel Structure

Each carpel consists of:

• a) Ovary: Ovoid, spherical, cylindrical, or elongated, in the form of a modified leaf sheath. Ovules originate within the ovary.
• b) Style: A filamentous prolongation of the ovary.
• c) Stigma: Situated at the end of the style. It has a wide shape and secretes a liquid that absorbs and adheres to pollen grains.

The ovule is a small, spherical or ovoid corpuscle. It joins the ovary wall by a filament called the funiculus, a bundle of vascular tissue. The site where the ovule joins is called the raphe. It has two membranes: an outer one, the primine, and an inner one, the secundine. Both are traversed by a hole called the micropyle and surround a cluster of cells called the nucellus.

Pollination and Fertilization

• Pollination is the transfer of pollen from the anther of a stamen to the stigma of a carpel. It can be natural or artificial.
• Fertilization is the union of the oosphere and antherozoid.

The ovule and pollen initiate fertilization. The ovule consists of two membranes that wrap a set of cells, one of which stands out: the embryo sac, inside which other cells are produced. After anther dehiscence (opening) and the transfer of the pollen grain to the stigma of a carpel, fertilization is initiated.

Fertilization Process

Pollen is attached to the stigma by:

• a) Roughness of the exine: This allows it to attach to the cells of the stigma, which secrete a sweet and viscous liquid that sticks to the pollen grain.
• b) The secretion of the stigma: It penetrates through the pores of the exine and is absorbed by the intine.

If the pollen grain is from the same plant species on which the stigma has been deposited, the liquid it absorbs will cause it to germinate. As the exine is inextensible, the intine protrudes through one of the pores of the exine and lengthens, forming the pollen tube.

Descent of the Pollen Tube

The pollen tube is inserted between the cells of the stigma, then between those of the style, and continues to grow until it progressively reaches the ovary. Meanwhile, it feeds on the protoplasm contained in the pollen. Behind the vegetative nucleus, the generative nucleus is inserted into the tube and divides into two cells, called antherozoids, while the pollen tube enters the ovule through the micropyle. When the tube enters the micropyle, it does so across the secundine. Once inside the ovule, the nucleus moves between the cells of the nucellus and makes contact with the membrane of the embryo sac.

Antherozoids in the Embryo Sac

In the embryo sac, the pollen tube releases the antherozoids. One of them goes to the oosphere's nucleus, and the other goes to the secondary nucleus.

Embryo and Endosperm Formation

The oosphere and antherozoid join, forming the zygote. The secondary nucleus joins with the other antherozoid to form the accessory zygote. The zygote and accessory zygote then multiply to form the embryo and the endosperm, respectively. The endosperm cells accumulate nutritive substances.

Seed and Fruit Formation

After fertilization, the ovule becomes the seed. If we consider that the seed is inside the fruit, and that the seed comes from an ovule that is inside an ovary, it follows that the fruit is the transformed ovary.

Leaf Structure

Veins

Sieve tubes and xylem vessels, which traverse the central cylinder, are inserted through the petiole into the leaf lamina. There, they branch out, forming veins. The function of veins is to conduct sap and form the supporting structure or skeleton of the plant. Leaves have venation patterns. Those with a single main vein are called uninerved, while those with several are called plurinerved. Plurinerved leaves can have veins that travel through the lamina without branching (simple-nerved) or that branch (reticulate-nerved). Simple-nerved leaves can be parallel-veined (when they run from the base to the tip in parallel) or curve-veined (when they follow the same route but curve concentrically).

Petiole

The petiole is the cylindrical portion that joins the lamina with the stem. The angle formed at the junction is called the axillary angle, where the bud is located. Sieve tubes and xylem vessels penetrate the petiole and branch out into the lamina, forming the veins. Leaves with petioles are called petiolate, and those without are called sessile.

Sheath

The sheath is the part of the leaf that is inserted into the stem. It is located at the lower end of the petiole or, in sessile leaves, at the base of the lamina. If the sheath wraps the stem completely, it is called sheathing; if it wraps it partially, it is called semi-sheathing.