Themes > Science > Botanical Sciences > Plant Reproduction and Development > Angiosperms Reproduction and Life Cycle











Angiosperms, the flowering plants, represent the most advanced condition among terrestrial plants. While their sexual organs -flowers- are unquestionably their crowning achievement, their origin remains a topic of considerable debate. There is general agreement that the precursor to the ancestoral flower was a modified whorl of leaves, and that extant flowers with multiple parts of indefinate number probably represent the ancestoral condition. For an interesting discussion on the origin of flowering plants, the reader is referred to Peter Crane, et.al. (1995) (See references).

The angiosperm life cycle possesses the following advances over conifers:

  • Reproductive structures are flowers rather than cones.
  • Ovules embedded in female sporophylls rather than lying bare on the surface
  • Gametophyte still further reduced
  • Double fertilization to produce a diploidtriploid endosperm nutritive material
  • Seeds enclosed in fruits that develop from the ovary or related structures

The Flower











In order to understand angiosperm reproduction and their life cycle, knowledge of floral anatomy is necessary. 

























Flowers may be perfect, containing both male and female structures, or imperfect, containing only one sex. Furthermore, they may be complete, possessing sepals, petals, stamens and pistils, or incomplete, lacking one or more of these structures. Sepals enclose and protect the remaining flower during the bud stage. The collection of sepals is called the calyx. Further interior lie the petals, which together comprise the corolla. The corolla is usually the showy part of the flower and serves to attract potential pollinators. The calyx and corolla are collectively known as the perianth. Inside the corolla lies the stamens, or male structures, which consist of pollen-bearing anthers that sit atop stalk-like filaments. Centermost on the flower is the female reproductive organ, or pistil, which may consist of one to several carpels. The carpel is composed of a stigma, where the pollen is received, a style upon which the stigma is borne, and a basal ovary. One to several ovules are contained within the ovary, and these contain the egg cells.

The Fruit












Fruits generally develop from the maturing ovary, the structure that houses the ovule, and later, the embryo. They have evolved to protect the seeds and to assist in their dispersal. As revealed in the above photographs, fruits have evolved a variety of forms. Fruit type is one characteristic used by taxonomists to catagorize different angiosperm species.

Fruits are classified as dry or fleshy (dehiscent or indehiscent), simple or compound. A simple fruit develops from a single pistil, while a compound fruit develops from several ovaries in a single flower (an aggregate fruit) or from several ovaries each from separate flowers (a multiple fruit). Blackberries are an example of an aggregate fruit. Pineapples are multiple fruits. Both are fleshy. A hazelnut is an example of a dry fruit.

The Life Cycle

The following diagram represents a hypothetical angiosperm life cycle:





































Arbitrarily begin the angiosperm life cycle with the development of the diploid flower on the mature sporophyte plant. Within the anther microsporocytes develop and undergo meiosis to produce haploid microspores. Each of these undergoes one mitotic division to yield a generative cell and a tube cell. These together comprise the immature microgametophyte, or pollen grain. The generative cell completes a second mitotic division to produce two sperm nuclei. Inside the ovule a single megasporocyte develops, undergoes meiosis, and produces four haploid megaspores. Three of these degenerate, while the fourth undergoes three mitotic divisions to produce an eight-nucleate embryo sac, or mature megagametophyte. Upon pollination, the pollen grain germinates on the stigma, a pollen tube grows down the style and into the ovary via the micropyle. One sperm nucleus fuses with the egg to create a diploid zygote, while the other sperm nucleus fuses with the two polar nuclei to produce the nutritive, triploid endosperm. This process of double fertilization is unique to flowering plants. The embryo develops inside the embryo sac, integuments of the ovule form a protective seed coat around it, and the mature ovary forms a protective fruit around the seed.

Eventually the seed is shed from the fruit and the embryo temporarily suspends development in order to overwinter. Upon receipt of the proper hormonal and environmental cues, seed germination occurs and the embryo grows into a mature diploid sporophyte which produces flowers to complete one cycle of the alternation of generations.


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