Ivan pavlov, a russian repeatedly exposed dogs to the sight of food and, at the same time, the sound of a at first, the dogs salivated because of the food; this reaction is known as but after a while, the dogs begin to salivate when they heard the sound, even if no food was present. this learned reaction is known as and the sound is called a

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.