Angiosperm life cycle

Within the anther of the flower, the male gametophyte develops from microspore mother cells, which divide meiotically, each giving rise to four haploid microspores. The nucleus in each microspore then divides mitotically, developing into a pollen grain containing two nuclei. One of the nuclei subsequently divides again, usually upon germination, resulting in three nuclei per pollen grain: two sperm nuclei and one vegetative nucleus.

Within the ovule, the female gametophyte develops from a megaspore mother cell, which divides meiotically to produce four haploid megaspores. Three of the megaspore degenerate; the fourth divides mitotically, developing into an embryo sac consisting of seven cells with a total of eight haploid nuclei (the large central cell contains two nuclei, the polar nuclei). One of the smaller cells, containing a single haploid nucleus, is the egg cell.

The pollen germinates on the stigma, producing a pollen tube that grows down the style into the ovary. The two sperm nuclei enter the embryo sac through the tube; one nucleus fertilizes the egg cell, the other merges with the polar nuclei, forming the triploid endosperm. The embryo undergoes its first stages of development while still within the ovary of the flower and the ovary itself mature to become a fruit. The seed, released from the mother sporophyte in a dormant form, eventually germinates forming a seedling.

Seeds and fruits come from flowers.

When you eat a lima bean, you are eating an embryo and other parts of a seed. A seed is a mature ovule. A new plant grows from a seed.

After fertilization, the fertilized egg develops rapidly and becomes the embryo of a seed. Two cotyledons develop, which often contain food for the growing embryo. Sometimes food is also stored outside the cotyledons. Meanwhile, the wall of the ovule becomes the seed coat. Dicot seeds, such as bean, have an embryo, two cotyledons, and a seed coat. Monocot seeds, such as corn, have one cotyledon, an embryo, and a seed coat.

Some seeds are made so that they can move easily from place to place. This helps in the spreading of new plants. Catalpa seeds are shaped like wings, and milkweed seeds have hairs that help the wind more them.

When you eat a peach, tomato or olive, you are eating a fruit. Following fertilization in the angiosperm, the ovary develops into the fruit. As the gymnosperms, the ovules develop into the seeds, each containing its embryo sporophyte. Fruits may develop from a single carpel or from fused carpels, and each carpel may contain one or more ovules. In the peach, for instance, the skin, the fleshy, edible portion of the fruit, and the stone are three layers of the matured ovary wall. The seed is inside the stone. This is an example of a simple seed formed from a single ovule. In the pea, the pod is the mature ovary wall, and the peas themselves are seeds. The raspberry is an aggregate of many tiny, fleshy fruits, each formed from a separate carpel.

Fruits provide new and ingenious way for seed dispersal. In some plants, the fruit itself carries wings, as in many of our common trees; in others the fruit bursts open, shooting out the seeds. Some species of geranium send forth their seeds by a sort of slingshot. Often, the fruits are edible and brightly colored, tempting birds and mammals to eat them. The seeds within the fruit pass unharmed through the digestive tract hours later and often miles away. Burrs adhere to fur, feathers or one’s trouser legs, to be carried by unwilling messenger to far-off fields and meadows. In the tumbleweed, the whole plant is blown across the open country, scattering seeds as it goes. Angiosperm seeds travel much father and faster than simpler seeds of the gymnosperm.

Date: 2014-12-22; view: 1011