Who in the hollow of his hand
Holds all the growth of all our land —

These lines come in the very last stanza of the poem The Heart of the Tree. The lines say that a man who plants a tree holds the growth of ‘all our land’ (the entire world, the human civilisation) in his hand. ‘Hollow of his hand’ means the palm of his hand where one is most protected. So, it brings a sense that the man doesn’t want to risk the growth of the land by allowing it to slip out of his hand. Rather he takes utmost care to ensure that the tree is planted.

In a different interpretation, his hands might be hollow (empty) with nothing else to give to his fellow men. But he has got a larger heart to show his love for the neighbours. That leads him to plant the tree which, in turn, makes a great contribution for the cause of humankind. The beneficial effects of this great work are often indirect and long-term. But the future generations are sure to reap the benefits.

Again, on a symbolic level, the act of plantation is an act of goodness — one from which many other acts of goodness will sprout. A helpful attitude will grow among people. So, plantation is an act that inspires people to do good to others. It has the potential to grow the civilization in its true sense.

Thus the poet glorifies the act of plantation and motivates the readers in planting trees.