First one is the old 'that vs which' - which I swear I'll never get my head around, no matter what clever ways I'm given to remember it. Should it be '... flowers tied to a tree that stands beside others.' or '... which stands beside others.' And secondly, what would be the clearest way to show / word the plural of X - as in when people write a line of kisses on a greeting card? eg: 'Only a line of exes / X's / Xs' It's for a poem so I need it to be instantly clear and obvious to the reader what the word is.
Disclaimer: non-native opinion ahead, apply with caution. "flowers tied to a tree that stands besides others" means that specific tree and no other "flowers tied to a tree, which stands beside others" means some tree that happens to stand beside others. Which one you use depends on whether the information stands beside others is optional or mandatory. Another example: "A house, which is red, was bought by Ann." Ann bought a house. That is all the important information and if you remove the which-clause, the meaning of the sentence doesn't change. The colour of the house is only additional information. "A house that is red was bought by Ann." Here it is important exactly what house Ann bought. A red one. It is suggested that there were more options and out of these, Ann bought the red one. If my explanation didn't help, this article does a good job at it: http://www.writersdigest.com/online-editor/which-vs-that On the second question, I'm not sure but I think I recall the Chicago Manual of Style recommending Xs for uppercase letters and x's for lowercase.
In the OP it looks like a restrictive clause. That’s what make the difference (at least in American English): https://en.oxforddictionaries.com/usage/that-or-which
Regarding the Xs (or X's), I would not use "exes". Seeing the actual word immediately reminds me of ex-boyfriends. Like, he was my ex. It also just looks damn weird. Xs or X's for me. Though I agree with Steerpike - the apostrophe is probably unnecessary. Xs' looks grammatically incorrect, like the Xs are in possession of something.
"exe" makes me think of the suffix of an executable program in Windows. That little tidbit aside, I would not naturally read it phonetically and it would be confusing if I encountered it while reading. I agree with steerpike: "Xs" is best. An apostrophe makes it seem like you're implying possession of some sort.
