................ represents a collection of living plants designed chiefly to illustrate relationships within plant groups.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a term that describes a collection of living plants, specifically one that is designed to illustrate relationships within plant groups.

2. **Identifying Key Terms**: The key terms in the question are "collection of living plants" and "illustrate relationships within plant groups". This indicates that we are looking for a specific type of institution or place that serves educational and scientific purposes.

3. **Exploring Relevant Concepts**: In biology, there are various institutions that deal with plants, such as nurseries, arboretums, and botanical gardens. Among these, a botanical garden is specifically known for its collection of living plants.

...