Why do only the hard parts of organisms generally leave fossils?

The soft parts usually degrade and break down easily while the hard parts remain for longer periods of times. This makes it so that the imprint gets out into the surface, making a fossil.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.