Hey,WW: I finally found a picture of the structure I was looking for. All I could think of was "ostia". That's the mouth of a river. The word I couldn't remember was "stomata". Here's a URL about it:
http://www.microscopy-uk.org.uk/index.html?http://www.microscopy-uk.org.uk/schools/images/stomata.html

Something is goofed up here I keep erasing and entering it over, and error persists. Try copying and pasting it.