Reproduction in lower and higher plants notes part 1.2

Sexual reproduction

Definition: The reproductiom in which there is formation and union of male and female gametes occurs is called sexual reproduction.

1. Involves fusion of two compatible gametes (male and female).

2. Takes place after certain maturity.

3. In higher plants, flowering indicates beginning of reproductive phase.

4. Production of genetically different offspring.

5. Variations are useful for survival and evolution of species.

6. Flowers, specialized reproductive structure producing haploid gametes-

7. Sexual reproduction – Two major events –

Meiosis-

Production of gametes (n):

i)Male gametes in anther

ii)Female gametes in ovule

Fusion of gametes (Fertilization):

i)Diploid zygote

ii)Embryo

iii)New plant-(2n) sporophyte

8. Diploid sporophyte is dominant plant body → Meiosis → Haploid spores →

gametes ← within flower ← Reduced structure ← gametophyte

9. Sexual reproduction are grouped into three distinct stages

i)Pre-fertilization 

ii)Fertilization

iii)Post-fertilization

Structure of flower

a) Male reproductive whorl of flower is called androcium.

b) Individual member of androcium, is called stomen

c) Stomen constist of filament connective and anther.

Anther

Structure of anther

i)Usually dithecous (Having two lobes) → tetrasporangiate (Having four pollen sacs)

ii)Monothecous (Having single lobe) → Bisporangiate (Having two pollen sacs)

iii)When young it is homogeneous.

Parenchymatous with epidermis.

iv)Heterogeneity appears with formation of archesporial cell.

T. S. of anther :

i)Mature anther has anther wall and microsporangium.

ii)Anther wall is outer layer and microsporangium has sporogenous tissue.

iii)Anther wall has four layers, viz. Outer epidermis (Protective) followed by fibrous endothecium, thin walled middle layers and tapetum innermost nutritive layer.

iv)Microsporangium contains microspore mother cells (2n) which undergo meiosis to form microspore tetrad.

v)Haploid (n) microspores separate from tetrad.

vi)When microspores develop wall around it then it is known as pollen grain.

vii)From archesporial cell primary parietal cell forms anther wall and sporogenous cell forms sporogenous tissue.