Describe how the male gametes in the pollen grain reach the female egg cell in the ovule.

Here are the solutions to the questions:

b) (i) Pollen grains are deposited on the stigma. Describe how the male gametes in the pollen grain reach the female egg cell in the ovule.

When a pollen grain lands on a compatible stigma, it absorbs sugary fluid from the stigma. This stimulates the pollen grain to germinate and grow a pollen tube. The pollen tube grows downwards through the style, secreting enzymes to digest the surrounding tissue, towards the ovary. The two male gametes (nuclei) travel down the pollen tube as it elongates. The pollen tube eventually enters the ovule through a small opening called the micropyle, allowing the male gametes to reach the female egg cell for fertilization.

b) (ii) The graph in Fig. 7.2 shows the changes in the length of a pollen tube over a 180 minute period.

The specific question for part (ii) is not fully visible in the image. However, based on the graph, we can observe the following trend:

The length of the pollen tube increases steadily over the 180-minute period.

• Initially, the growth is relatively slow, with the length reaching approximately $1.4 \text{ mm}$ in the first $60 \text{ minutes}$.
• The growth rate then increases significantly between $60 \text{ minutes}$ and $120 \text{ minutes}$, where the length increases from $1.4 \text{ mm}$ to $4.6 \text{ mm}$.
• After $120 \text{ minutes}$, the growth rate slows down again, with the pollen tube reaching a length of approximately $5.3 \text{ mm}$ by $180 \text{ minutes}$.