There are tiny breathing pores located on the underside of the leaf, called stomata. They guide CO² into contact with the water. This is where the photosynthesis occurs. The photosynthesis process requires water taken up by the roots through the main stem and into the leaves. There it meets up with Carbon dioxide. In order for the photosynthesis process to occur the leafâ??s interior tissue must be kept moist. The stomata open and close to regulate the flow of moisture, preventing the leaf from drying out. The stoma also allows the outflow of water vapor and wasted oxygen. The stomata are very important to the plantâ??s well being and must be kept clean to promote vigorous growth. Dirty or worse yet clogged stomata are about equal to you breathing with a bag or cloth over your head. You can breath but itâ??s labored, you worked harder and waist energy that you could have used to do something more productive.

That's why you don't blow smoke on your plants.

But it's your choice.