Contexts where a comma means “and”

A comma is commonly used as a short form of the word and in newspaper headlines. In what other contexts is this convention common?

This question came to mind as I was trying to parse the following clause from “A Visit from St. Nicholas”. The comma between obstacle and mount seems like it must represent the word and.

As dry leaves before the wild hurricane fly,
When they meet with an obstacle, mount to the sky;
So up to the house-top the coursers they flew,
With the sleigh full of toys — and St. Nicholas too:

As it's poetry, I don't think it's entirely realistic to apply normal standard of deconstruction here.

Personally, I have no problem with the earlier version as quoted by OP (without the word "that"). Maybe Clement Clarke Moore didn't either - I don't know if it was his idea to change it, or some pettifogging editor/publisher. But it parses fine to me as...

As dry leaves before the wild hurricane fly,
[As dry leaves] [flying thusly] when they meet with an obstacle, mount to the sky;

The second line is just a poetic expansion/restatement of the first. Both describe how the leaves move, as referred to by "So" (In such a manner) on the next line. It's unnecessary (but not absolutely wrong) to include "that" (I could have included it before flying thusly too if I'd wanted).

I disagree with other answers saying "when they meet with an obstacle" is "parenthetical". You should be able to remove a parenthetical clause without radically affecting the sense of what remains, but obviously the whole reason the leaves "mount to the sky" is because they've come up against an obstacle. If there were no obstacle, they'd just bowl along close to the ground.

No, the comma between obstacle and mount doesn't represent and. When they meet with an obstacle is a parenthetical remark inserted for rhythm and meter as well as metaphorical value, so it has to be set off with commas. Put into normal sentence order, the sentence becomes: Just as dry leaves that meet an obstacle fly into the sky when they're being blown about in a hurricane, the reindeer flew up to the housetop with St. Nicholas and the sleigh full of toys. Too prosaic to be interesting, but that's one version of how the sentence runs in normal, boring expository prose.

In a list of three or more items, the commas stand for and: I bought peanut butter, bread, jelly, and bananas for lunch. The first two commas stand for and but the third stands for itself: the Oxford or serial comma, even though many writers can't stand it at all.

The comma after obstacle does not stand for the word and. The phrase “when they meet with an obstacle” is parenthetical* and is marked front and back with commas.

The inverted word order used in many places in the poem creates some confusion. Also complicating the issue is that early versions of the poem contain a grammatical error. Where you have:

As dry leaves before the wild hurricane fly,
When they meet with an obstacle, mount to the sky;
So up to the house-top the coursers they flew,

Later versions read:

As dry leaves that before the wild hurricane fly,
When they meet with an obstacle, mount to the sky;
So up to the house-top the coursers they flew,

It may help to see the text prosaically reordered and the parenthetical phrase marked with parentheses:

As dry leaves that … fly before the wild hurricane
… Mount to the sky (when they meet with an obstacle);
So … the coursers they flew up to the house-top,

Wikisource describes the poem’s history and reproduces the published variants of the text.¹

Footnotes

* This statement has been challenged, so here I explain why – though its absence would be detrimental to the meaning of the verse – the phrase “when they meet with an obstacle” is quite properly described as parenthetical: it interrupts the grammar of the sentence, and is set apart in print with commas as befits its importance.

The dictionary describes a parenthesis as “[a] word, phrase, or sentence, by way of comment or explanation, inserted in, or attached to, a sentence which would be grammatically complete without it [emphasis added]”.² That is, a parenthesis contributes to the semantics (meaning) of the sentence but interrupts and stands beside (independent of) the syntax (grammar, structure) of the sentence. Wikipedia derives the word from the Greek παρένθεσις, meaning “to place alongside of”.³

In print, a parenthesis is set apart from the enclosing sentence using punctuation. When it is not an important insertion, square brackets or curved lines called parentheses are commonly used; for greater importance or emphasis, commas or even dashes are used.⁴

(Bold face is used in this note to mark parenthetical text.)