Man, a tree growing from Brahma - Yajnavalkya

As a tree of the forest, 

Just so, surely, is man. 

His hairs are leaves. 

His skin the outer bark. 

From his skin blood, 

Sap from the bark flows forth. 

From him when pierced there comes forth 

A stream, as from the tree when struck. 

His pieces of flesh are under-layers of wood. 

The fibre is muscle-like, strong. 

The bones are the wood within. 

The marrow is made resembling pith. 

A tree, when it is felled, grows up 

From the root, more new again; 

A mortal, when cut down by death — 

From what root does he grow up?

Say not 'from semen'

For that is produced from the living, 

As the tree, forsooth, springing from seed, 

Clearly arises without having died. 

If with its roots they should pull up 

The tree, it would not come into being again. 

A mortal, when cut down by death — 

From what root does he grow up? 

When born, indeed, he is not born [again]. 

Who would again beget him? 

Brahma is knowledge, is bliss, 

The final goal of the giver of offerings, 

Of him, too, who stands still and knows It.

from the Ninth Brahmana of the Third Adhyaya (chapter 3.9)

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Yajnavalkya [Yagyavalkya] (c. 700 BC) India

Translated by Robert Ernest Hume

Source: The Thirteen Principal Upanishads by Robert Ernest Hume, Oxford University Press, 1921