To understand how water moves in our world, attention must be directed to the hydrologic cycle. The hydrologic cycle is essentially a water continuum, representing the different paths through which water circulates and is transformed in the natural environment (Fig. 1). Being a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth s surface, particularly the oceans, is evaporated into a gaseous form and enters the atmosphere as water vapor (clouds). The atmospheric moisture is eventually returned to the Earth s surface in the form of rain or snow.
Cell differentiation. In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Different tissues have different kinds of cells.
Thus, different cells can have very different physical characteristics despite having the same genome. A cell that can differentiate into all cell types of the adult organism is known as pluripotent. Such cells are called embryonic stem cells in animals and meristematic cells in higher plants.ach cell type is defined by its particular pattern of regulated gene expression. Cell differentiation is thus a transition of a cell from one cell type to another and it involves a switch from one pattern of gene expression to another.