Flowering plant
The amount and complexity of tissue-formation in flowering plants far exceeds that found in Gymnosperms. The
vascular bundles of the stem are arranged such that the
xylem and
phloem stand side by side on the same radius. In the Dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue, known as cambium; by the formation of a layer of cambium between the bundles (interfascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from it by the development of xylem on the inside and phloem on the outside. The soft phloem soon becomes crushed, but the hard wood persists, and forms the great bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings. In the smaller group, the Monocotyledons, the bundles are more numerous in the young stem and scattered through the ground tissue. Moreover they contain no cambium and the stem once formed increases in diameter only in exceptional cases.
Flowers
The characteristic feature of angiosperms is the flower, which shows remarkable variation in form and elaboration, and provides the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is that of ensuring fertilization of the ovule and development of
fruit containing
seeds. The floral apparatus may arise terminally on a shoot or from the axil of a leaf. Occasionally, as in violet, a flower arises singly in the axil of an ordinary foliage-leaf. However, more typically, the flower-bearing portion of the plant is sharply distinguished from the foliage-bearing or vegetative portion, and forms a more or less elaborate branch-system called an
inflorescence.
The reproductive cells produced by flowers are of two kinds,
microspores or
pollen grains, are the "male" cells and are borne in the
stamens (or microsporophylls), and the "female" cells called
megaspores, in which the egg-cell is developed, contained in the
ovule and enclosed in the
carpel (or megasporophyll). The flower may consist only of these parts, as in
willow, where each flower comprises only a few stamens or two carpels. Usually, however, other structures are present and serve both to protect the sporophylls and to form an envelope attractive to pollinating insects. The individual members of these surrounding structures are called
sepals and
petals (or
tepals in flowers such as
Magnolia where sepals and petals are not distinguishable from each other). The outer series (calyx of sepals) is usually green and leaf-like, and functions to protect the rest of the flower, especially in the bud. The inner series (corolla of petals) is generally white or brightly coloured, and more delicate in structure, and functions in attracting a particular insect or bird by agency of which pollination is effected. This attraction involves colour and scent, and frequently also nectar which is secreted in some part of the flower. These characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans.
While the majority of flowers are
perfect or
hermaphrodite (having both male and female parts in the same flower structure), flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization.
Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal
pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel).
Homomorphic flowers may employ a biochemical (physiological) mechanism called
self-incompatibility to discriminate between self- and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers.
Fertilization and embryogenesis
Double fertilization refers to a process in flowering plants during
reproduction, in which two
sperm cells fertilize two
cells in the
ovary. The
pollen grain adheres to the stigma of the
carpel (female reproductive structure) and grows a
pollen tube that penetrates the
ovum through a tiny pore called a
micropyle. Two sperm cells are released into the ovary through this tube. One of the two sperm cells fertilizes the egg cell, forming a
diploid zygote or embryo, also called the
ovule. The other sperm cell fuses with two haploid polar nuclei in the center of the
embryo sac. The resulting cell is
triploid (3n). This triploid cell divides through
mitosis and forms the endosperm, a nutrient-rich
tissue inside the
fruit. When seed develops without fertilization, the process is known as
apomixis.
Fruit and seed
As the development of embryo and endosperm proceeds within the embryo-sac, its wall enlarges and commonly absorbs the substance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument to form the
seed-coat; or the whole nucellus and even the integument may be absorbed. The ovary wall has developed to form the
fruit or pericarp, the structure of which is closely associated with the manner of distribution of the seed. Frequently the influence of fertilization is felt beyond the ovary, and other parts of the flower take part in the formation of the fruit, as the floral receptacle in the
apple,
strawberry and others. The character of the seed-coat bears a definite relation to that of the fruit. Their function is the twofold one of protecting the embryo and of aiding in dissemination; they may also directly promote germination. If the fruit is a dehiscent one and the seed is therefore soon exposed, the seed-coat has to provide for the protection of the embryo and may also have to secure dissemination. On the other hand, indehiscent fruits discharge these functions for the embryo, and the seed-coat is only slightly developed.
Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families of flowering plants, the
Poaceae, or grass family, is by far the most important, providing the bulk of all feedstocks (
rice, corn (
maize),
wheat,
barley,
rye,
oats,
millet,
sugar cane,
sorghum), with the
Fabaceae, or legume family, in second place. Also of high importance are the
Solanaceae, or nightshade family (
potatoes,
tomatoes, and
peppers, among others), the
Cucurbitaceae, or
gourd family (also including
pumpkins and
melons), the
Brassicaceae, or
mustard plant family (including
rapeseed and
cabbage), and the
Apiaceae, or
parsley family. Many of our fruits come from the
Rutaceae, or rue family, and the
Rosaceae (rose family, including
apples,
pears,
cherries,
apricots,
plums, etc).
In some parts of the world, certain single species assume paramount importance because of their variety of uses. An example is the coconut (
Cocos nucifera) on Pacific
atolls. Another example is the olive (
Olea europaea) in the
Mediterranean.
Flowering plants also provide economic resources in the form of
wood,
paper, fiber (
cotton,
flax, and
hemp, among others), medicines (
digitalis,
camphor), decorative and landscaping plants, and many, many other uses.
*
List of flowers*Angiosperm Phylogeny Group (2003). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II.
Botanical Journal of the Linnean Society 141: 399-436.
Available online.
*
Angiosperms – Tree of Life Web Project
*Cronquist, Arthur. (1981)
An Integrated System of Classification of Flowering Plants. Columbia Univ. Press, New York.
*
Oldest Known Flowering Plants Identified By Genes, William J. Cromie, Harvard Gazette, December 16, 1999.
* Stevens, P.F. (2001 onwards).
Angiosperm Phylogeny Website at Missouri Botanical Garden.
*
L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. http://delta-intkey.com
