Roots are positively gravitropic, usually growing downward and stems are negatively gravitropic, usually growing upward. Auxins are probably responsible for this growth. Chemotropism - a response to chemicals. The growth of a pollen tube is in response to chemicals produced produced by the plant ovary. Hydrotropism - a response to water.
Most plants have a positive response to water.
Critical length is the length of daylight above or below which a species of plant will flower. Long-day plants flower only when exposed to day lengths longer than their critical length. These are usually late spring and early summer flowers. Short-day plants flower only when exposed to day lengths shorter than their critical length. These are usually early spring and fall flowers.