Chapter 1: Section 7: Page 58: Line 26 (1007)

 They strolled among the elm-shadows, eating sandwiches and apples out of paper bags, "a peripatetic picnic," as the Professor called it, slipping thereupon into his lecture-hall style.

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"elm-shadows"
Elm trees create plenty of shade and are one of the most popular trees to be used in public landscaping, often planted as boundary markers for paths, creating tunnel-like effects when planted to either side of a path. The elm is both a symbol of pastoral peace and death. Perhaps Heino and Merle are walking in the shadows of these border trees because they are, technologically speaking, crossing from a time of pastoral peace, where technology has begun to make impacts in our lives and seen as improving them, toward a time when technology will begin its decent from the peak of improvement, hurtling toward the ground as it becomes destructive and deadly, until it finally crashes beyond the zero into nuclear annihilation.
    Or maybe its just a bit of descriptive tat.

"eating sandwiches and apples"
Apples being the symbolic fruit of which Eve partook. Yeah, I know the fruit is never actually described but instead of being a pedantic nerd on the Internet, how about understanding the cultural symbols all around you. There's a reason the Apple logo is that of an apple with one bite taken out of it. And it's not because it's the mark of the beast and an indication of man's inability to resist temptation! It's about gaining knowledge! Maybe those are the same things though. See my comments on "elm-shadows."

"out of paper bags"
They didn't have plastic bags yet! If you want to learn more about the history of plastics, Pynchon covers that topic in Gravity's Rainbow.

"a peripatetic picnic"
This is a pun. The Professor is commenting on how the picnic is a traveling picnic but also it's a picnic where they discuss philosophical matters as they eat. So peripatetic as the adjective in that they're traveling but also as in The Peripatetic school founded by Aristotle, as we see Heino begins to lecture Merle as they walk.