Why are protist so difficult to classify?

Mostly because the 'Kingdom Protista ' is defined by what they are NOT. (e.g. not plants, fungi, or animals). Most are single-celled, some are colonial, and some multicellular. Some are photoautotrophic, some ingestive heterotrophs, and some capable of both modes of nutrition. In other words, there is a lot of diversity between 'protists' - some are closer to animals, and some are closer to plants. Research in molecular biology has helped clear up some of the confusion, but protist classification is an ongoing process.

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.

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.