Reproduction in flowering plants , Structure & functions of the flower
The flowering plants are a large group of seed plants that called Angiospermae , as their seeds develop within a pericarp , They are common in various habitats and vary in the size from small herbs to giant trees , The flowering plants reproduce by specialized organ which called the flower .
Flower is the reproductive organ in the flowering plants and it is a short stem whose leaves are modified to form the various floral parts , Bract is a leaf that from its axil the flower emerges , It varies in the shape and colour from a plant to another ( scale , green ……… ) .
The flower may be :
- With a bract or without a bract .
- Stalked ( carried on a pedicel ) or Sessile ( not caaried on a pedicel ) .
Origin of the flowers
Solitary flowers : which may be Apical , so that it stops the growth of the stem as in tulip flower or Axial that doesn’t stop the growth of the stem as in Petunia flower .
Grouped flower : Flowers arise grouped on the floral axis in various aggregations which called inflorescences as in the flower of bean and manthur , Inflorescence is a group of flowers on the floral axis in various aggregations .
Structure of the flower
The typical or complete flower ( hermaphrodite ) such as in bean , apple , onion and Petunia has four floral whorls , where the leaves of each whorl alternate with those of the next whorl as follows :
Calyx ( outer whorl of the flower ) consists of green leaves that called sepals , It protects the inner parts of the flower againt drought or rain or wind .
Corolla ( follows the calyx from inside ) consists of one row or more of coloured leaves that called petals , It protects the floral sexual parts , It attracts insects for the occurrence of pollination process .
Androecium ( flower male organ ) consists of numerous leaves that called stamens , where each one of them consists of filament that carries an apical anther , anther contains four sacs of pollen grains , It produces the pollen grains ( male gametes ) .
Gynoecium ( flower female organ and lies in its centre ) produces the ovules ( female gamete ) , It consists of one carpel or more that may fuse or remain separated , where each one of them consists of :
- Ovary : the swelling base of the carpel and contains the ovules .
- Style : a thin neck that attaches over the ovary and ends with the stigma .
- Stigma : a sticky disc where the pollen grains adhere .
In the flowers of most monocotyledon plants as tulip and onion , the leaves of calyx are difficult to be differentiated from those of the corolla , and so the two outer whorls are called perianth .
Functions of the flower
The flower performs its functions in reproduction for the species continuity and this requires the following :
- Pollen grains formation .
- Ovules formation .
- Pollination and fertilization.
- Fruits and seeds formation.
Pollen grains formation
If you examine a T.S. in a mature anther of a large-sized stamen as in lily plant , you find that the anther consists of four sacs of pollen grains where the pollen grains are formed as the following :
- During the flower development and before the formation of pollen grains , these sacs are full of large-nucleated diploid cells ( 2n ) called the mother spore cells .
- Each cell of these mother spore cells divides meiotically , forming four haploid ( n ) cells called the microspores .
- The nucleus of the microspore divides mitotically into two nuclei , where one of them is called the tube nucleus and the other called the generative nucleus , therefore the pollen grain is formed , then its wall becomes thick for its protection .
- The anther becomes mature and the separating wall between each two adjacent pollen sacs degenerates , then the sacs open and the pollen grains become ready for dispersal .
During the formation of pollen grains in the anther , parallel changes oocur in the ovary as the following :
The ovule starts to appear as a simple swelling on the interior wall of the ovary and it contains a large mother spore cell ( 2n ) .
During the ovule growth : A funicle develops which connects the ovule with the ovary wall and through which the nutrients reach it , Two integuments develop which surround the ovule completely , except at a minute hole called the micropyle through which the ovule is fertilized .
The mother spore cell ( 2n ) divides inside the ovule meiotically , giving a row of four haploid cells ( n ) , Three of these cells degenerate and the fourth remains and grows rapidly , forming the embryonic sac which is surrounded by a nutritive tissue called the nucellus , Inside the embryonic sac , many stages occur as follows :
- The nucleus divides mitotically for three times , giving 8 nuclei , where each four of them migrate to one pole of the embryonic sac .
- From each four nuclei , one nucleus moves to the centre of the embryonic sac which are called the two polar nuclei ( embryonic sac two nuclei ) .
- Each nucleus of the remaining three nuclei at each of the embryonic sac poles becomes enveloped by some cytoplasm and a thin membrane , forming distinct cells .
- The middle cell of the three cells that are close to the micropyle grows to become the egg , while the two side cells are called synergids and the three cells that are distant from the micropyle are called the antipodal cells .
The ovule becomes ready for fertilization .