Two hundred years ago, we did not have the problem that we have today dealing with Global Warming and the Ozone layer. As time progressed we began to develop more and more problems with the environment as the amount of pollution in the air increased. I believe that it is due to this that the density of stomata per unit of leaf decreased. Pollution harmed and mutated certain characteristics of plants, making the stomata not as dense as it was two hundredd years ago.