Botanists generally recognize four major divisions of plant species, and only the most recently evolved produces flowers. Within the flower, you can observe the life cycle of a flowering plant in its entirety, from the development of sex cells to pollination and fertilization to the production and distribution of the seeds.
Flowering plants evolved from the first seed-bearing plants, the gymnosperms, which include species such as conifers and cycad palms. The earliest seed plants produced pollen that, carried upon the wind, fertilized exposed seeds hanging on cones. Flowers developed 132 million years ago and granted extra protection to the seeds and, with their coloration and scent, also attracted pollinators. The development of flowers naturally created shifts in the life cycle as well.
Producing flowers obviously requires energy expenditures for the plant, so there must be some significant benefit imparted to flowering plants. Flowers serve to remove the aspect of chance from the flowering plant's life cycle. Non-flowering plants rely to a significant degree on happenstance: a gust of wind in the right direction or a rainstorm at an opportune time. By attracting pollinators, the flowering plant better guaranteed successful cross-pollination and reproduction, a benefit reflected in the fact that, despite their relatively recent evolution, 65 percent of plant species are flowering plants.
A plant begins life at germination, slowly unfolding from the seed and, through the process of cellular division, maturing into a full-grown plant. At some point during its life cycle, the flowering plant develops flowers. In most species, flowers contain both male and female parts. Within these structures, cells divide to form eggs or sperm. Eggs are enclosed in the rudimentary seed, located in the plant's ovary at the base of the flower, while sperm cells develop within pollen grains.
The female structure of the flower is called the pistil, and it generally protrudes from the center of the flower. Pollen carried, either by the wind or with the help of a pollinator, sticks to the pistil. One cell divides to extend a tube into the pistil, extending toward the ovary, while the other cells develop into two sperm cells. When the pollen tube reaches the ovary, one sperm fuses with egg while the other fuses with other female cells, producing nutritive content to feed the seedling as it grows.
Each ovule inside of the ovary contains one egg cell, and each develops into a seed. The walls between ovules develop tough seed coats that protect the embryonic plant. In many flower species, the ovary also grows an extra layer called the pericarp or the fruit coat. This fleshy, sweet covering entices animals to eat it. The hard seed coating protects the seed as it passes through the animal's digestive tract, and when it is deposited in its droppings, the seed lies waiting for germination, ready to begin the next generation.