The correct option is Chemotropism.
Explanation
Tropic movements, or tropisms, are directional growth movements in plants occurring in response to an external stimulus. The direction of the growth is determined by the direction of the stimulus. These movements are classified based on the nature of the stimulus, such as chemicals, light, gravity, or water.Analysis of Options
- Chemotropism (Correct): This is the growth movement of a plant part in response to a chemical stimulus. In angiosperms, the growth of the pollen tube down the style towards the ovule is induced by specific chemical substances (such as sugars and peptones) secreted by the synergids present in the ovule. Since the movement is directed by a chemical gradient, it is classified as chemotropism.
- Hydrotropism (Incorrect): This refers to the growth of plant parts, typically roots, towards water or moisture.
- Phototropism (Incorrect): This is the directional growth of a plant part in response to light. For example, shoots generally grow towards a light source (positive phototropism).
- Geotropism (Incorrect): Also known as gravitropism, this refers to growth in response to gravity. Roots typically show positive geotropism (growing downwards), while shoots show negative geotropism (growing upwards).
Key Takeaway
The pollen tube exhibits chemotropism as its growth through the style is guided by chemical signals exuded by the ovule to facilitate fertilization.