The effect of the words quoted from The Gardener 85 by Rabindranath Tagore is the following: The personification suggests that the happiness of the past is eternal.
In the poem, Rabindranath wishes to share the flowers and happines from the sping he is living with the reader, even if he reads it a hundred years after he writes the poem itself. Therefore he states "In the joy of your heart may you feel the living joy that sang one spring morning, sending its glad voice across an hundred years", suggesting that the joy in the air on the moment he wrote the poem will last for so long that any reader at any time (even a hundred years later) could be able to feel it.