Examine Thoreau’s representations of the natural world in “Spring” and “Conclusion.” What lessons does he glean from nature? What main messages does he attempt to transmit to readers? And where we can see these?

In Walden, the chapter on “Spring” includes detailed observations of changes, from tiny to huge, that Henry David Thoreau notices in the natural world. On a symbolic level, Thoreau employs elements of nature to support his ideas.
For example, observing Walden Pond, he notes such things as the changing temperature of the water as the ice melts, the effects on its inhabitants of the sound his axe makes when he strikes the ice, and the specific animals that appear to drink the water when the ice melts. Paying attention to the foliage emerging on a hillside, the author meditates on the larger abilities of Nature: “One hillside illustrated the principle of all the operations of Nature. The Maker of this earth but patented a leaf.” He goes on make numerous comparisons between the natural world and the creative work of writing, including Egyptian hieroglyphics, mythology, and poetry. Nature helps him understand the earth in totality.

The earth is not a mere fragment of dead history, stratum upon stratum like the leaves of a book, to be studied by geologists and antiquaries chiefly, but living poetry like the leaves of a tree, which precede flowers and fruit.

The arrival of spring seems to Thoreau like the human possibility of awareness and optimism, as opposed to looking backward with regret. He appreciates spring as the time when people, like the natural world to which they belong, can find renewal and growth—an attitude that he strongly encourages—and even forgiveness for the sins for which they constantly try to atone.

We should be blessed if we lived in the present always, and took advantage of every accident that befell us, like the grass which confesses the influence of the slightest dew that falls on it; and did not spend our time in atoning for the neglect of past opportunities, which we call doing our duty. We loiter in winter while it is already spring. In a pleasant spring morning all men’s sins are forgiven.

In the “Conclusion,” Thoreau uses elements of his previous observations about nature to make statements about human nature, history, and future possible endeavors. One seeks the natural phenomena of other places really only to disguise the search for essential inner meanings, he muses. Thoreau draws on natural elements found in different parts of the world to raise suggestions for such ventures and encourages his reader to think of the interior states that voyages represent.

One hastens to Southern Africa to chase the giraffe; but surely that is not the game he would be after. How long, pray, would a man hunt giraffes if he could? . . .
[I]t is easier to sail many thousand miles through cold and storm and cannibals, in a government ship, with five hundred men and boys to assist one, than it is to explore the private sea, the Atlantic and Pacific Ocean of one’s being alone.

Thoreau wraps up the chapter, and the book, by drawing connections from the smallest item to the largest realms of philosophy. Seeing a bug, he thinks of God.

As I stand over the insect crawling amid the pine needles on the forest floor, and endeavoring to conceal itself from my sight, and ask myself why it will cherish those humble thoughts, and bide its head from me who might perhaps be its benefactor, and impart to its race some cheering information, I am reminded of the greater Benefactor and Intelligence that stands over me the human insect.
http://digitalthoreau.org/walden/fluid/text/17.html