Assertion : The leaves of larkspur in juvenile stage are different from those in mature stage.

Reason : Heterophylly in larkspur is an example of plasticity.

The assertion is true. The leaves of larkspur (Delphinium species) show heterophylly, where the leaves in juvenile and mature stages differ in shape and structure. 

The reason is also true; heterophylly is an example of plasticity, where a single plant displays different leaf forms in response to environmental changes or developmental stages.

Both (A) and (R) are true and (R) doesn’t explain (A).