________is essential constituent of all living things.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the essential constituent of all living things. This means we need to identify the fundamental element that is a part of every living organism.

2. **Identifying the Element**: In biology and chemistry, the element that is universally recognized as a key building block of life is carbon. It is found in all organic compounds and is crucial for the structure and function of living organisms.

3. **Examples of Carbon in Living Organisms**:
- **Plants**: Carbon is a major component of plant cells. For instance, the leaves of plants contain carbon in the form of carbohydrates, proteins, and other organic molecules.
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