Question

What joy the unborn eyes shall see

Answer 1

The speaker mentions that planting a tree today would give fruits in coming days. Our next generations would be delighted seeing so much vegetation and reap its benefits. So all the credit goes to the man who plants a tree.

"Unborn eyes" refers to the future generations and "joy" refers to the happiness and benefit they will receive from the tree in future.

Hope it helps you :)