From the PRL archives:

I think that I shall never see a matrix lovely as a tree. — Trees, by Guy L. Steele Jr., MIT, 1973

You might recognize the opening line from Joyce Kilmer’s 1914 poem Trees, or from Radia Perlman’s Algorhyme (published 1985).

The poem is online in at least one other place, but the copy linked above (from BYTE magazine) comes with a footnote on How this poem came to be printed.