|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:
|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.
|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
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.