How many chromosomes are present in the nucleus of a sperm cell? Which of the following reactions seen in plants is an example of positive phototropism?

Here are the solutions to the questions:

1. How many chromosomes are present in the nucleus of a sperm cell?

A human somatic (body) cell contains 46 chromosomes. A sperm cell is a gamete (sex cell), which is haploid, meaning it contains half the number of chromosomes of a somatic cell.

$$\frac{46}{2} = 23$$

Therefore, a human sperm cell contains 23 chromosomes.

The correct option is D.
$\boxed{\text{D. 23}}$

2. Which of the following reactions seen in plants is an example of positive phototropism?

• Phototropism is the growth of a plant in response to light.
• Positive phototropism means growing towards the light.
• Negative phototropism means growing away from the light.
• Plant stems typically exhibit positive phototropism, growing towards a light source to maximize photosynthesis.

Let's analyze the options:

• A. a plant root which grows away from the light: This is negative phototropism (for roots).
• B. a plant root which grows downwards towards gravity: This is positive geotropism.
• C. a plant stem which grows towards the light: This is positive phototropism.
• D. a plant stem which grows upwards away from gravity: This is negative geotropism.

The correct option is C.
$\boxed{\text{C. a plant stem which grows towards the light}}$