Correct Option
The correct option is Length.
Explanation
Growth in plants is defined as an irreversible permanent increase in size of an organ or its parts. Since growth is a quantitative phenomenon, it can be measured using various parameters depending on the nature and structure of the plant part involved. Common parameters include increase in fresh weight, dry weight, length, area, volume, and cell number.
Detailed Analysis
- Length: Growth in structures that elongate primarily in one direction is best expressed in terms of length. The pollen tube is a tubular structure that grows apically through the style to reach the ovule. Therefore, its growth is measured by the increase in its length.
- Area: This parameter is used for flat, laminar structures. For example, the growth of a dorsiventral leaf is best expressed in terms of an increase in surface area.
- Number: This parameter is used when individual units increase in count. For example, the growth of single-celled organisms (like bacteria or yeast) or cells in a culture is measured by the increase in cell number.
- Weight/Volume: These are general parameters often used for fruits or storage organs (e.g., a watermelon cell can increase in size by up to 3,50,000 times, which is an increase in volume/size).
Key Takeaway: Different plant structures require specific metrics for measuring growth: Pollen tubes are measured by length, leaves by surface area, and microbial cultures by cell number.