Sukkah 11: God's Rain Cloud

Climbing plants on a sukkah - ivy? Grapes? Etc. When they are attached to the ground, they can't really be part of the schach, unless they were overwhelmed in quantity by schach that was not attached to the ground. Also, tzitzit! In parallel to sukkah. In this case, can a string that is tied to two corners be acceptable for tzitzit? Plus, the 4 species make an appearance here. Do they need to be bound together? What if the myrtle branch has berries? And finally, what is the source for the requirement that schach comes from that which cannot be rendered impure? With the accompanying verses - how the cloud provides God's protection on this earth, and the water in that cloud is not rendered impure. It's from the very beginning of creation!