Pollination

pollen sacs, which is where the pollen is formed. There are nectaries or a single nectary at the base of the stamens that secrete the food reward for the pollinators.

In the middle of the flower are the carpels, collectively called the gynoecium. These are the female parts of the flower in which the megasporangia form. They enclose the ovules that have an egg inside. The ovule, after fertilization, will mature into a seed with the carpel maturing into a fruit. These carpels and fruit are unique to the angiosperm type of plant.

A perfect bisexual flower will have both stamens and carpels, while a unisexual or imperfect flower will have no stamens (called carpellate) or no carpels (called staminate). Some plants will have staminate and carpellate flowers on the same plant and are monoecious. Species that have separate plants with staminate and carpellate flowers are called dioecious.

The receptacle is the stem to which the flower or inflorescence is attached with the peduncle being the stalk of a flower. The calyx or cluster of sepals most resemble the leaves because they are green. Sepals can be separate (polysepalous) or fused (synsepalous), forming a tube rather than separate sepals. The corolla (or petals) of the flower will attract insects and birds for pollination).

POLLINATION

Pollination involves the transfer of pollen from the anthers to the stigma of the same plant species so that there can be germination and growth of the pollen tube to the ovule. There can be self-pollination in some species or cross-pollination. Pollen transfer happens by water, wind, insects, birds, and other animals. Most of the animal crosspollination happens because of bees. They feed on the nectar and pollen so they can pollinate one or more species of plant. Plants that have adapted to this will have a flower that has a landing platform for the bees. These flowers are often yellow or blue in color in order to attract the bees.

The first phase of pollination involves the landing of a pollen grain on a stigma of a receptive plant. It has glandular tissue that nourishes the pollen tube as it elongates and grows down the style of the flower. Flowers have a chemical mechanism that prevents

self-fertilization. It makes it impossible to have a flower pollinate itself in most cases. The pollen tube enters the ovule in the carpel and penetrates one of the sterile cells on either side of the egg, called synergids. These will degenerate and lead to fertilization of the egg. Figure 45 shows the important structures in fertilization:

When a pollen grain reaches the tip of the carpel, called the stigma, it germinates into a pollen tube. It forms two sperm cells that travel down the pollen tube into the style, the micropyle, and into the ovule chamber. The pollen tube ruptures, and the sperm cell fertilizes the egg, while the other sperm cell fuses into the polar nuclei, forming what’s called the endosperm nucleus.

The fertilization of the egg inside the carpel by a pollen grain leads to seed formation in the carpel. The formation of fruit without the egg being fertilized is known as parthenocarpy. A fruit is nothing more than a ripened ovary, which differentiates it from a vegetable, because a vegetable does not have reproductive organs. This is what makes squash, eggplant, and tomatoes fruit versus carrots and turnips, which are modified roots. Potatoes, ginger, and onions are modified stems. Figs and pineapple are the combined gynoecia of more than one flower (or a whole inflorescence).