Re: Why do plants vary stomata density not size?

Hi Liz,

I didn’t find an authoritative answer to this, perhaps because it’s very difficult to prove why 
something didn’t evolve.  However, I have a couple ideas that may explain your observations.
One way to think about this is to look at how stomata are formed.  As you probably know, 
stomata are pores formed by two guard cells.  The guard cells are formed by the asymmetric 
division of pre-existing epidermal cells.  You can see a cartoon version of this here: http://faculty.washington.edu/ktorii/stomata.html

Therefore, the size of the guard cells is limited by the size of the parent epidermal cells.  It 
follows that if the size of the epidermal cells is fairly constant, so too will the size of the guard 
cells.  Of course, you might imagine that the guard cells could grow after they are formed, but 
this presents a couple problems:  

First of all, it may be difficult for a guard cell to increase appreciably in size, since it is 
embedded in the plant epidermis, which is made of a sheet of fairly rigid cells.  Plant cells can 
and do expand during growth, but when this is happening, it is usually a whole group of cells 
expanding together.  Mechanically, it is much more problematic for a single cell to expand while 
surrounded by rigid, non expanding cells.

Second of all, even if the guard cells did grow in size, opening a large stomata may be 
problematic.  Stomata open and close as the guard cells change their shape.  You can see a 
cartoon version here: http://click4biology.info/c4b/9/plant9.2.htm#7  
Even though the surrounding cells are somewhat rigid, they must deform somewhat as the guard 
cells expand and the stomata opens.  However, the bigger the stomata, the bigger the guard 
cells would have to be, and the more they would have to deform neighboring cells in order to 
fully open the stomata.  This clearly makes it more difficult to open a larger stomata.

So, for the case of stomata, it seems to me that making more stomata is a solution that presents 
fewer problems than making larger stomata. 

Alex Brands