(a) Describe the processes that occur from the time a pollen grain is deposited on the stigma of a flower to the . formation of a zygote. (b) Explain the im...

(a) From pollination to the formation of a zygote

When a pollen grain is deposited on the stigma, it absorbs sugary fluid and germinates, growing out a pollen tube. The pollen tube grows down through the tissues of the style towards the ovary, its direction controlled by chemical substances (chemotropism) and its path softened by enzymes it secretes. The tube nucleus leads at the tip, while the generative nucleus divides to form two male gametes. The tube reaches an ovule and enters it through the tiny opening called the micropyle. The tip of the tube then bursts and releases the two male gametes into the embryo sac. One male gamete fuses with the egg cell (fertilisation) to form the zygote (2n), which later develops into the embryo. The second male gamete fuses with the two polar nuclei to form the endosperm nucleus (3n). This union of two gametes is called double fertilisation.

(b) Importance of the amniotic sac to the mammalian embryo

• It is filled with amniotic fluid that acts as a shock absorber, protecting the embryo from mechanical injury and pressure.
• It keeps the embryo moist and prevents it from drying up.
• It allows the embryo to move freely and grow evenly.
• It helps to maintain a constant temperature and pressure around the embryo.

(c) Classification of the diseases by causative organism