It is shown that the response of transpiration to light is different in species from Gramineae and Cyperaceae compared to species from other families. Blue light (380–500 nm) causes both a rapid response (rise time 2 min) and a slow response (rise time 5 to 10 min) in species from these two families but induces only a slow response in other families. Red light results only in a slow response in all plants investigated.

It is hypothesized that the rapid response to blue light is due to the special anatomy and/or function of the so-called grass-like (graminaceous) stomata of plants belonging to Gramineae or Cyperaceae.