170 THE BIOLOGY OF FLOWERING PLANTS 



of the greatest interest, the details of which should be 

 studied in his important memoir. We may take a few 

 illustrative examples. He studied his plants in the humid 

 spring climate of Minnesota and the hot dry summer of 

 Salt Lake City. He found that they could be divided into 

 three groups as regards stomatal behaviour. 



(i) Cereals. In dry conditions the stomata of wheat, 

 barley, and oats remain closed for long periods, showing 



100;/ 



i-50 



Fig. 17. — Transpiration and stomatal movement; the thick lines 

 indicate transpiration (T/E) for the half-hour periods ; the continuous 

 line indicates the per cent, opening of the stomata. (After Knight, 

 modified.) 



only a slight and transitory opening in the morning. In 

 greenhouse conditions, with higher atmospheric humidity 

 and better water supply, they show a daily opening of several 

 hours, but only of limited extent. We have already noted 

 that the grass stoma is relatively inactive. In our own 

 climate the opening is probably of longer duration. It 

 would be of interest to know the assimilation relations of such 

 plants in arid conditions. The maize has a partial opening 



