During fertilization, the direction in which the pollen tube grows is controlled by

Analyze the question: It asks what controls the direction of growth of the pollen tube after a pollen grain lands on the stigma.

Concept: After pollination, the pollen grain germinates on the stigma and produces a pollen tube. The tube grows down through the style to the ovary, enters the ovule through the micropyle, and delivers the male gametes.

Inside the pollen grain there are two nuclei:

• the tube nucleus (also called the vegetative nucleus), which directs and controls the growth of the pollen tube
• the generative nucleus, which divides to form the two male gametes

Step by step:

1. The pollen grain lands on a compatible stigma and absorbs nutrients and water.
2. The tube nucleus controls the growth of the tube, so it moves to the tip of the tube.
3. It guides the tube down the style toward the micropyle. Chemical signals from the ovule (chemotropism) help with this, but the control of growth lies in the tube nucleus.
4. The generative nucleus divides into two male gametes, which follow behind the tube nucleus.
5. When the tube reaches the micropyle, the tube nucleus disintegrates and the male gametes are released.

Checking the options:

• A. Ovary: it houses the ovules and does not control the tube's growth.
• B. Ovule: it releases chemical attractants, but it is not the controlling structure.
• C. Pollen grains: too general. The control comes from a specific nucleus inside.
• D. Pollen tube nucleus: it controls the direction and growth of the tube.
• E. Style: it is only the pathway the tube grows through.

Tip: Remember "tube nucleus = tube boss." It directs the tube, while the generative nucleus is the one that divides to make the male gametes.