NEET Biology Notes Plant Kingdom Angiosperms
The earliest system of classification used only gross superficial morphological characters such as habit, colour, number and shape of leaves, etc. Such systems were artificial. The natural systems developed were based on natural affinities among the organisms and consider not only the external features but also internal features, like anatomy, embryology and phytochemistry. The most accepted system was given by George Bentham and Joseph Dalton Hooker.
At present, phylogenetic classification systems based on evolutionary relationships between the various organisms are acceptable
- In angiosperms, the seeds are enclosed by fruits. The pollen grains and ovules are developed in special structures called flowers. They are found in wide range of habitats. Angiosperms (or the flowering plants) constitute the largest group with about 12,500 genera and 250000 species that occupy a very wide range of ecological habitats.
- Their size varies from tiny (Wolffia) to tall trees like Eucalyptus. The male sex organs in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The female sex organs in a flower is the pistil or carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed as embryo sac. The embryo sac formation is preceded by meiosis. Each embryo sac has a three-celled apparatus, i.e. one egg cell and two synergids, three antipodal cells and two polar nuclei.
- The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule. The pollen tubes enter the embryo sac, where two male gametes are discharged. One of the male gamete fuses with the egg cell to’ form a zygote. The other male gamete fuses with the diploid secondary nucleus to produce the triploid Primary Endosperm Nucleus (PEN). Because of the involvement of two fusions, this event is called double fertilisation, a unique event in angiosperms.
- Zygote , develops into an embryo and the PEN develops into endosperm, which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilisation. During these events, the ovules develop into seeds and the ovaries develop into fruit. Angiosperms provide food, fodder, fuel, medicines and several other important products.
They are divided into two classes:
- Dicotyledons (two cotyledons in seeds)
- Monocotyledons (single cotyledon in seeds)
Plants possess two cotyledons in their seeds, reticulate venation in leaves, phyllotaxy varied, vascular bundles open and present in a ring, secondary growth can occur. Dicotyledons have three sub-classes:
- Polypetalae Sepals and petals are distinct, petals free.
e.g. mustard, pea, etc.
- Gamopetalae Sepals and petals are distinct, petals are partially or completely fused.
e.g. sunflower, potato, etc.
- Monochlamydeae Incomplete due to the non-distinction of sepals and petals.
These possess one cotyledons, parallel venation in leaves, phyllotaxy alternate or spiral, vascular bundles scattered and closed (due to the absence of combine) adventitious roots very common. Examples of monocotyledons are onion, grasses, etc.