hgablid wrote:

The answer is clear to me but for further understanding I have a question. How can be represent standard deviation on the graph of normal distribution?

Standard Deviation is a number. It is one particular value, a parameter (characteristic) of a distribution. The less it is, the more data tend to the mean.

Of course, there is no point in plotting that particular number on the graph, however, in case of the normal distribution, we can plot what it represents.

We know that about 68% of the data lie within one standard deviation of the mean, 95% – within two, 99.7% – within three. So we can show these areas on the graph, where 68%, 95%, 99.7% of the data lie.

About 34% of the seed pods are within one standard deviation on the either side of the mean (together they make 34% + 34% = 68%). Another bars to the left and right will bring us additional 13.5% of the seed pods each (13.5% + 34% + 34% + 13.5% = 95%) and so on.

In other words, we can say that about 68% of the seed pods are between 2.75 and 3.25 inches in diameter. Similarly with 95%, 99.7%.