Vines

Vines are non-solid blocks that were added in Update 0.9.0. They act similar to ladders, except they do not need to be placed on the side of a block. Vines can stick to any block or specific ones. Vines can spread around surface to surface if left alone. Vines with Cobblestone can be crafted into Mossy Cobblestone through the stonecutter.

Trivia

 * Vines grow on many different Trees, although they are more common on Oak and Jungle trees.
 * Vines have different hit boxes than ladders, so you can't climb from vine to ladder.
 * Vines are similar to Mycelium, both spread block to block.
 * Vines can spread on nearly any block, except bedrock.
 * Vines cannot be climbed if they are not adjoining a block, you will just walk through.