There usually is some degree of variability in a sample (golfers do not use only titleist balls; students do not always eat at taco bell). the is a measure of the variability in a sampling distribution.

There are several measure to describe the distribution of some observations. The mean for example give an idea about the average observation. The measure that gives us an idea about variability is the standard deviation; Intuitively it measures how much observations differ from the average.

Leaves specifically the gas exchange between plants and the environment. while plants undergo photosynthesis, they also partake in cellular respiration. respiration is the process that happens in the leaves of plants.