38 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



and completely by 3 a. m. After 3 a. m. the onion stomata began to 

 open very slowly to 20 per cent at 5 a. m. As a result of sunrise, 

 however, they opened within the next hour to an unstable maximum 

 of 80 per cent for the morning. This maximum continued with 

 several fluctuations until 11 a. m. when the stomata closed to 60 

 per cent, as in the preceding day. In alfalfa the stomata were 

 closed until after 4 a. m., when they began to open slowly, reaching 

 10 per cent at 5 a. m., and then more rapidly to 80 per cent at 8 a. m. 

 They continued in this condition for an hour, and then closed to 60 

 per cent at 10 a. m. By 11 a. m. they had closed completely. 



60 



T\ 



9 10 II NOON I 23*56789 10 II MT. I 334-56783 10 II 



Fio. 20. Series 16, showing movement in stomata of onion (A) and upper stomata 



of alfalfa (B). 



The stomatal movement hi onion resembles that of alfalfa in a 

 number of respects. Both open and close and exhibit day closure 

 and night opening at much the same time. On the other hand, the 

 stomata of alfalfa showed a much greater day closure and a corre- 

 spondingly greater night opening. The leaves of onion have a 

 larger amount of water relatively and are less affected by the factors 

 of evaporation in consequence, with the result that the stomata show 

 less day closure. The speed of day closure and secondary opening is 

 greater in the stomata of alfalfa, as well as the degree of opening. 

 The curve of stomatal movement in onion is more irregular than 

 that in alfalfa, as the stomata open or close rapidly for a time, then 

 slowly or not at all, and then more rapidly again. The forenoon 

 maximum of 80 per cent on July 27 is referred to as unstable, because 

 of a similar irregularity. The stomata opened to 80 per cent at 

 6 a. m., closed to 60 per cent the next hour, opened again to 80 per 

 cent at 8 a. m., closed to 70 per cent at 9 a. m. and opened once more 

 to 80 per cent at 10 a. m. Such behavior occurred to some extent 

 in all plants under certain conditions, but most commonly in succu- 

 lent plants, such as onion and Portulaca oleracea, and to a lesser 

 extent in sugar-beet, cow-beet, Rumex patentia, salsify, cabbage, and 

 others. However, under certain conditions, such as high evapora- 

 tion counteracted by high water-content, alfalfa, sweet-clover, corn, 



