The angiosperms or true flowering plants are presently the most dominant plants on Earth, comprising more than 95% of all existing plants (embryophytes or land plants). They also represent most of the agricultural and food crops (Kesseler and Stuppy 2009; Simpson 2010) . It is just proper therefore that this group of plants be accorded special concern.
These plants consist largely of the monocotyledons and the eudicots. Combined, they comprise 246,000 described species, equivalent to 97% of all angiosperms as of 2010 (Simpson 2010). Mathematical derivation indicates that there are more than 250,000 species of angiosperms.
These flowering plants exhibit wide diversity. They occur in various sizes, forms, growth habit, life span, and habitat preference. Their sizes range from less than 1 mm in diameter, as in the smallest flowering plant Wolffia, to more than 90 meters or 300 feet in height as in the eucalyptus trees of Australia. Wolffia is a water surface dweller and produces the smallest flowers. Although orchids may not appear so extraordinary, they produce seeds which can be less than 0.2 mm wide (waynesword.palomar.edu 2012), the smallest seeds.
It is widely known that the biggest single flower belongs to Rafflesia arnoldi, a parasitic plant which emits a rotten smell. To biologists, an important angiosperm is the thale cress (Arabidopsis thaliana) of the mustard family or Brassicaceae, often referred to as a laboratory “guinea pig” because it is widely used as an experimental plant.
In contrast to earlier plants (bryophytes and fern allies), the flowering plants have completely evolved true roots, stems, and leaves. But their greatest evolutionary innovations are the flower, which attracts active pollinators, and the fruit. To man and agriculture, the flowers and fruits in the angiosperms offer distinct advantages over the gymnosperms.
Certain flowers are now commercially produced for various purposes such as for ornamentation, as source of essential oils for perfumery, or even as food or food ingredient. In crop agriculture, cereals are cultivated for the harvesting of grains or kernels which are fruits technically called caryopsis. Orchards are established for the growing of fruit crops which produce different types of edible fruits with varying taste. Without flowers fruits will not exist.
Fossilized pollen grains indicate that the angiosperms evolved at least 140 million years ago during the Cretaceous period (65-142 million years ago) (Simpson 2010). Prior to their establishment, the dominant vegetation consisted of the flowerless gymnosperms, ferns and allies, and bryophytes. Obviously the landscape then would have a dull color, mainly green.
With gymnosperms, it was sufficient that there is wind to serve as vector of pollen grain transfer from the male to female cones. It was not necessary to have colorful parts to attract biotic pollinators. However, the exact landing of pollen grains on the ovule in the female cone is a hit-or-miss and so plenty of pollen grains must be produced by plants in order to ensure pollination and the formation of every seed. Dr. Frits W. Went aptly described this wasteful process as thus: Therefore billions upon billions of pollen grains must be produced to ensure that just a few of them will reach the right spot (Went and The Editors of Life 1963).
The coming of angiosperms made a vibrant change on the plant scenery, animal activity, and the process of pollination. But the first flowers must have been simple and inconspicuous like that of the Archaefructus liaoningensis the fossil of which was discovered in sediments 130 million years old in China.
With the advent of the flowering plants about 130 million years ago, pollination by insects must have likewise evolved, a case of coevolution. Insects land or burrow on flowers and move to another, collecting and dispersing pollen grains in the process. But it took another 30 to 40 million years, or between 90 and 100 million years ago, for the petals to evolve (Klesius 2002).
There are now flowers having petals of various colors, scent and sizes. They attract insects, birds, and other animals which serve as unknowing agents of pollination as they move from one flower to another. Some flowers emit rotten odor, but the same attracts pollinators. In orchids, some oncidiums have devoloped a uniquely ingenious way of attracting bees by producing flowers which mimic a swarm of bees (McKinley 2005). As reward, some flowers secrete nectar which the pollinators harvest. The pollen itself is harvested by bees and other animals as food. The Australian honey possum in fact feed on nectar and pollen, its sole source of nutrition (Klesius 2002).
The process of pollination therefore became more efficient. It is not purely a hit-or-miss anymore, as in pollination by wind alone. Wind pollination still exists, but animal-aided pollination accomplished more. The flowers, with their showy, colorful, or otherwise specialized features, act as beacons for the animal pollinators to collect and disperse pollen grains.
Further, pollination results to the formation not only of the seed, as in the gymnosperms, but also the fruit through double fertilization. With fruit, seed development is even more ensured and its natural dispersal became more improved.
In agriculture, understanding of this association between animals and plants has dramatically influenced the application of technology and choice of inputs. Total eradication of insects is no longer considered as an exacting strategy in crop protection. The effect of pesticides on honeybee population has become a subject of vigorous research, both in relation to crop production and honey production. Biodiversity conservation has found its niche in environmental agriculture.
(Ben G. Bareja, Oct. 2012)
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