Corrected sentences (two good choices):
1. The jungle is thick and dark, so orchids grow on tree trunks to get more sun.
2. The jungle is thick and dark. Orchids grow on tree trunks to get more sun.
Why these are correct (step by step):
- Both original parts are complete thoughts: "The jungle is thick and dark" and "orchids grow on tree trunks to get more sun."
- If you join them into one sentence with "so," put a comma before "so" because each part can stand alone. That makes sentence 1 correct.
- If you make them two sentences, start the second with a capital letter and drop the word "So." That makes sentence 2 correct.
- "To get more sun" is a short phrase that tells why the orchids grow on trunks. It is used correctly at the end.
Both options are fine. Use the first to show cause and effect in one sentence, or the second if you want two short sentences.