21.6 Angiosperms  (Page 2/3)

Fruit

As the seed develops, the walls of the ovary thicken and form the fruit. The seed forms in an ovary, which also enlarges as the seeds grow. In botany, a fertilized and fully grown, ripened ovary is a fruit. Many foods commonly called vegetables are actually fruit. Eggplants, zucchini, string beans, and bell peppers are all technically fruit because they contain seeds and are derived from the thick ovary tissue. Acorns (a nut), and winged maple whirligigs (whose botanical name is samara) are also fruit. Botanists classify fruit into more than two dozen different categories, only a few of which are actually fleshy and sweet.

Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts are examples of dry fruit. Another distinction is that not all fruits are derived from the ovary. For instance, strawberries are derived from the receptacle and in apples the receptacle forms the pericarp or the fleshy part. Some fruits are derived from separate ovaries in a single flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally, some fruits, like watermelon and orange, have rinds. Regardless of how they are formed, fruits are an agent of seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal. Wind carries the light dry fruit of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with color or perfume, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces. Other fruits have burs and hooks to cling to fur and hitch rides on animals.

The life cycle of an angiosperm

The sporophyte phase is the main phase of an angiosperm’s life cycle ( [link] ). Like gymnosperms, angiosperms are heterosporous. Therefore, they generate microspores, which will generate pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anthers’ microsporangia, male gametophytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.

Within the ovule, sheltered within the ovary of the carpel, a cell undergoes meiosis, generating four megaspores—three small and one large. Only the large megaspore survives; it divides to produce the female gametophyte. The megaspore divides three times to form an eight-cell stage. Three of these cells migrate to each pole of the embryo sac while two come to the equator. These two cells will fuse to form one central cell with two haploid polar nuclei. The two cells closest to the egg are called synergids; the three cells on the opposite end of the gametophyte are called antipodals.