Hi Susan, already gave you a great answer for (1), so I'll address (2) and (3).
(2) "Lacking haustoria" is not a gerund (nominal) phrase. Rather, it is an adjectival phrase that modifies "epiphytes."
The roots of "epiphytes" lack haustoria.
(3) Commas are used here because "parenthetical information" (that is, additional information) intervenes between the subject and its verb.
As an example, think of the "baseline" sentence: The apple is delicious.
Now if you want to add a piece of "parenthetical" information about the apple (e.g., you bought the apple at the grocery store), then you set commons around the parenthetical thought.
The apple, which I bought at the grocery store, is delicious.
English often breaks complex sentences down with commas to make the structure more apparent.