Secondary growth is observed in dicots and gymnosperm. It is not observed in pteridophytes and rarely observed in monocots. Secondary growth results in increase in girth or diameter of the stem by formation of secondary tissue by the activity of lateral meristem. So for study of secondary growth, teak (angiosperm) and pine (gymnosperm) are best suited. In wheat, fern and sugarcane there is no secondary growth

The critical study of secondary growth in plants refers to the examination of the process by which plants increase in girth or thickness due to the activity of lateral meristems. Secondary growth is primarily observed in woody plants and is responsible for the growth of the stem, branches, and roots. To conduct a critical study of this process, it is important to select suitable plant species for analysis.

Among the given options, the pair of teak and pine is the most suitable choice for studying secondary growth in plants. Here's why:

1. Woody Plant Characteristics:
- Both teak (Tectona grandis) and pine (Pinus spp.) are woody plants, known for their long and durable stems.
- Woody plants exhibit secondary growth, which involves the formation of secondary tissues (such as vascular cambium and cork cambium) that contribute to an increase in stem girth.

2. Lateral Meristem Activity:
- The study of secondary growth requires the examination of lateral meristems, specifically the vascular cambium and cork cambium.
- Teak and pine are both known to possess well-developed vascular cambiums, which produce secondary xylem (wood) and secondary phloem.
- The activity of the vascular cambium in teak and pine can be observed and analyzed to understand the process of secondary growth.

3. Commercial Importance:
- Teak and pine are economically significant plant species.
- Teak wood is highly valued for its durability, resistance to decay, and aesthetic appeal, making it a popular choice for furniture, flooring, and boat construction.
- Pine wood is widely used in construction, furniture making, and the production of paper and pulp.
- Studying secondary growth in teak and pine can provide valuable insights into the development and properties of economically important wood.

In conclusion, the pair of teak and pine is the most suitable choice for a critical study of secondary growth in plants. These woody plant species possess well-developed lateral meristems and exhibit significant economic importance, making them ideal candidates for understanding the process of secondary growth and its implications.