ISA Certified Arborist Practice Exam 2025 – All-in-One Resource for Comprehensive Exam Success!

Leaves are the primary site of photosynthesis in trees and other plants. This process occurs in specialized cells within the leaves called chloroplasts, which contain chlorophyll, the pigment responsible for capturing sunlight. During photosynthesis, leaves convert sunlight, carbon dioxide from the air, and water absorbed by the roots into glucose and oxygen. The abundance of surface area in leaves allows for maximized light absorption, making them especially efficient at this essential process.

Roots, while critical to a tree's overall health, are primarily responsible for anchoring the plant and absorbing water and nutrients from the soil rather than engaging in photosynthesis. The trunk serves as a supportive structure and transports water and nutrients between the roots and the leaves but does not play a direct role in the photosynthetic process. Bark, the outer layer of the trunk, protects the tree and also does not participate in photosynthesis, as it does not contain the necessary chloroplasts. Thus, the role of leaves is crucial for photosynthesis, making them the correct answer.