Sticky Substance

What plants?? Aphids maybe... A wash with mild detergent and water if the plant has leaves that can take it.

Could be a form of mite which can leave a sticky substance, and what appears to be sticky webs around the growth tips. Again wash with a mild detergent. If its a house plant and you dont mind chemicals use Xybol. Kills all beasties dead!

Could it be the stuff slugs and snails leave behind.....I am suffering badly with them in my garden at the moment and all this rain isn't helping either!