The process of random movement of individual molecules from a region of higher concentration to a region of lower concentration without semipermeable membrane is called the diffusion. Larger the difference in concentration, more rapid the flow of molecules. Diffusion is more rapid in gases than in liquids. When there is no further movement of molecules, a state of equilibrium is reached, which means that the diffusing substance is evenly distributed. For example the supply of carbon dioxide from atmosphere to the leaves for photosynthesis, and the loss of water from the leaves to the atmosphere.

The word 'chlorosis' is derived from a Greek word khloros meaning greenish-yellow or pale-green. The disease is caused due to the deficiency of magnesium . The magnesium is a macronutrient. The deficiency of magnesium causes lack of the green pigment called the chlorophyll and results into the yellow coloured leaves. If chlorosis is left untreated, it results in: Stunted growth and premature leaf abscission Low flower and fruit yield Increased susceptibility to pests and diseases