Angiospermae shows alternation of generation. Both sporophyte and gametophyte generations alternate with each other.
Alternation of Generation in Class Angiospermae
Sporophyte is a dominant generation. The adult plant of angiosperm is diploid sporophyte. It is differentiated into roots, stem and leaves. It produces flower at maturity. A flower is a modified shoot (stem and leaves). It consists of a pedicel, thalamus or torus and floral leaves (sepal, petal, stamens and carpals). The thalamus and these floral leaves, especially the stem and carpet, are so modified that they do not look like stems and leaves. Flower has following floral parts:
• Sepal and Petals
The sepals and petals are non-essential or non-reproductive parts of the flower. They protect the stamens and carpals. They also attract insects for pollination. After fertilization they may fall off.
Stamens and carpals are the essential or reproductive parts of the flower. The stamen consists of a filament and anther. The anther consists of pollen sacs. Pollen (microspores) are produced in the pollen sac by meiosis.
The carpel is female reproductive part of a flower. A carpel is composed of three parts: ovary, style and stigma. Ovary is a basal broader part of the carpel. Style forms a tube like structure. Stigma is the terminal part of carpel. The ovary contains ovule. The ovule consists of an outer integument (covering) and inner nucellus tissue.
The gametophyte is an inconspicuous and haploid (1n) generation. There are separate male and female gametophytes:
• Male Gametophyte
After pollination, the pollen grain is transferred to the stigma. Here it germinates to from pollen tube. The nucleus of the pollen grain divides by mitosis to form two male gametes and the tube nucleus. At this stage of development, the pollen grain is called male gametophyte.
• Female Gametophyte
Certain changes occur in ovule. It forms megaspore. The megaspore develops into female gametophyte. The female gametophyte consists of seven cells. One of these cells is called egg or oosphere.
The pollen tube grows through the style. It enters the ovule and then reaches the female gametophyte. Here it discharges the male gametes. One male gamete fuses with the egg to form oosphere. The second male gamete fuses with the secondary nucleus to form endosperm nucleus. So it is called double fertilization.
The oospore develops into an embryo. The endosperm nucleus develops into a multicellular nutritive tissue called endosperm. Meanwhile, the integument of the ovule forms testa and tegmen. The ovary wall develops into fruit. Seed usually undergoes a period of rest. It germinates in suitable conditions and produces a seedling .This seedling gradually changes into a sporophyte.
Double fertilization is a special process found in Angiosperms. In this case, two male gametes fuse with two female gametes simultaneously.
- A haploid (1n) male gamete fuses with haploid egg to form diploid zygote (2n). This zygote later develops into embryo.
- The second haploid (1n) gamete fuses with fusion nucleus (2n) and form triploid (3n) endosperm cell. This endosperm cell later develops into food storing endosperm tissue.
Double fertilization is an important evolutionary advancement. In this case, storage food is made only on fertilization or formation of zygote. It helps the plant to economize on its food resources.
Classification of Angiospermae
They have one cotyledon. The plants included in Moncotyledonae are called Monocotyledonous plants.
They have two cotyledons. The plants that are included in the Dicotyledonae are called Dicotyledonous plants.
Comparison between Dicot and Monocot Stems
|They have two cotyledons in the seed.||They have single cotyledon in the seed.|
|The number of sepal or petals are 4, 5 or multiple of 4, 5.||The number sepals or petals of are of 3 or multiple of 3.|
|The vascular bundles are present in rings.||The vascular bundles are scattered in the stem.|
|The veins in leaves from a net.||They have parallel veins in the leaves.|
|They may be herbaceous or woody.||They are always herbaceous.|
|The symmetry of flower may be regular or irregular.||The symmetry of flower also may be regular or irregular.|
|The petals may be polypetalous (separated petals) or gamopetalous (petals fused).||The petals may be polypetalous (separate petals) or gamopetalous (petals fused).|
|The ovary may be superior or ovary may be inferior.||The ovary may be superior or ovary may be inferior.|
Some Angiospermic families are described below:
- Rosaceae (Rose family)
- Solanaceae (Potato family)
- Fabaceae (Pea family)
- Caesalpiniaceae (Cassia family)
- Mimosaceae (Acacia family)
- Poaceae (Grass family)