Every year, you can observe the life cycle of plants in your garden. As spring weather warms, the first brave leaflets unfurl from branches and seedlings press up from the earth. Soon, the landscape is lush with green foliage, and the flowers begin to bloom. Pollen torments allergy sufferers, and seeds whirl down from trees or carry along on the breeze. Winter's barrenness only closes the cycle, holding promise of another year of plenty to come.
The plants are an ancient kingdom and, through many ages of evolution, have developed different varieties distinguished from each other, in part, by differences in their life cycle. Botanists recognize four major divisions of plants: mosses, ferns, nonflowering seed plants and flowering plants. As each type evolved, small changes to its life cycle allowed it to better survive the Earth's changing climate and conditions.
Alternation of Generations
All members of the plant kingdom exhibit alternation of generations, a term that refers to a life cycle that includes two distinct physical forms distinguished based on the chromosome count inside of the cells. The sporophyte generation contains a full set of chromosomes, while the gametophyte generation contains only half the number as the sporophyte generation. The gametophyte generation produces the plant's sex cells. As retired biology professor John W. Kimball explains, primitive plants primarily exist in the gametophyte stage, while this stage is inconspicuous in most later-evolved plants.
Mosses and ferns represent the more primitive plant forms and exhibit slightly different life cycles than the more familiar seed plants. Mosses primarily display the gametophyte generation with half the number of chromosomes. This generation produces sex cells that, carried to each other by water, fuse into the sporophyte generation. The sporophyte generation develops to produce spores, from which new moss plants grow. In ferns, a separate gametophyte structure called the prothallus produces sex cells, which fuse and grow into the familiar fern plant, the sporophyte generation. Ferns develop spores on the undersides of their leaves that, borne on the wind, develop into new prothalli.
Seed-producing plants have a different life cycle than their spore-producing predecessors. Unlike mosses and, to a lesser degree, ferns, the sporophyte generation dominates, with the gametophyte generation much harder to spot. Seed plants produce pollen, which contains sperm cells, and female structures that contain egg cells. In flowers, the small structures that produce pollen and egg cells constitute the gametophyte generation. After pollination and fertilization occurs, plants produce seeds, which grow into a new season of plants.
Understanding the life cycles of your plants helps you to better care for and understand them, whether saving seed from a favorite flower or choosing a shade tree based on seed and fruit mess. Also, because reproductive structures are often developed independently of normal plant growth, understanding how they develop can help you to maximize the blooms and fruit your plants produce. As Edward B. Rayburn of the West Virginia University Extension indicates, plants devote energy last toward reproduction--flowers, spores, seeds and fruits--so supporting this stage of the life cycle with proper care will result in enjoyable, productive plants.