The leaves  have a green pigment called chlorophyll which helps leaves  to capture the energy of the sunlight and  to synthesis food. Other parts of the plants  do not have chlorophyll therefore they are unable  to synthesise food.

What is autotrophic nutrition?
Autotrophic nutrition is the process by which organisms synthesize their own food using inorganic substances, such as carbon dioxide and water, with the help of sunlight or other forms of energy. Autotrophs, also known as producers, include plants, algae, and some bacteria. They are able to produce organic compounds, such as glucose, through the process of photosynthesis.

What is heterotrophic nutrition?
Heterotrophic nutrition is the process by which organisms obtain their food by consuming other living organisms or organic matter. Heterotrophs, also known as consumers, include animals, fungi, and some bacteria. They are unable to synthesize their own food and rely on plants or other animals for their nutrition.

Why can leaves synthesize but other parts of plants cannot?
Leaves are specialized structures in plants that are primarily responsible for photosynthesis. They have several adaptations that make them efficient in synthesizing food. Here are some reasons why leaves can synthesize while other parts of plants cannot:

1. Chloroplasts: Leaves contain numerous chloroplasts, which are the organelles responsible for photosynthesis. Chloroplasts contain the pigment chlorophyll, which captures sunlight and converts it into chemical energy.

2. Large surface area: Leaves have a large surface area that allows them to capture a maximum amount of sunlight. The broad, flat shape of leaves also helps in providing a larger area for the exchange of gases, especially carbon dioxide, which is required for photosynthesis.

3. Vascular system: Leaves are connected to the rest of the plant through a complex network of veins that transport water and nutrients. This vascular system ensures that the necessary raw materials for photosynthesis, such as water and minerals, are supplied to the leaves.

4. Stomata: Leaves have small openings called stomata, which are present on the underside of the leaf. Stomata allow for the exchange of gases, including carbon dioxide and oxygen. Carbon dioxide enters the leaf through stomata and is used for photosynthesis.

5. Mesophyll cells: Leaves have specialized cells called mesophyll cells, which are responsible for carrying out photosynthesis. These cells contain a high concentration of chloroplasts and are well adapted for the efficient capture of sunlight.

Conclusion:
Leaves are the primary sites of photosynthesis in plants due to their specialized structures and adaptations. They possess chloroplasts, a large surface area, a vascular system, stomata, and mesophyll cells, all of which contribute to their ability to synthesize food through the process of photosynthesis. Other parts of plants, such as stems and roots, have different functions and are not primarily involved in photosynthesis.