DAILY MOVEMENT IN ONION. 37 



essentially the same manner, although not to the same degree. The 

 outstanding difference lies in the fact that the stomata of sugar- 

 beet are alike, while those of alfalfa differ on the two surfaces of 



the leaves. 



THE DAILY MOVEMENT IN ONION. 



As the leaves of onion are essentially hollow cylinders, with no 

 distinct upper and lower epiderm, but one set of epidermal strips 

 was collected in each major series that included this plant. However, 

 one special series 6 hours in length was made to find whether the 

 difference in lighting on the different sides of the leaf had any effect 

 upon the* behavior of the stomata. In addition, a number of iso- 

 lated tests were made for the same purpose. The results showed 

 that the difference of lighting did cause variation in the stomata 

 affected at certain times, and under certain conditions especially. 

 On this account, at each stripping during a 24-hour series, three to 

 four strips were collected from the different sides of the leaf, so that 

 the average degree of opening for that leaf could be found. 



There are almost as many stomata in onion epiderm as unspecial- 

 ized epidermal cells, as the end of each such cell is usually separated 

 from the next by a stoma (plate 4). However, because of the size 

 of these cells, there are about the same number of stomata to a 

 square millimeter of epiderm as in a sugar-beet leaf. The stomata 

 are rather large and unusually simple in form and mechanism. 



The general type of behavior found in the stomata of onion is 

 shown in series 16, started 9 a. m. July 26, and ending 11 a. m. July 

 27, 1916. As usual, a set of alfalfa epiderm was collected in the 

 same series for comparison. Because of the high humidity, neither 

 the onion nor the alfalfa stomata showed the degree of day closure 

 that would be expected in view of the high temperature and brilliant 

 sunshine (fig. 18). At the outset, the onion stomata were 70 per 

 cent open and those of alfalfa 80 per cent. At 10 a. m. both species 

 had stomata 80 per cent open, and these began to close immediately. 

 By the following hour, alfalfa showed complete closure and remained 

 closed until 3 p. m. The stomata of onion closed much more gradu- 

 ally, reaching 40 per cent at 1 p. m., when they opened as gradually 

 to the maximum at 5 p. m. The alfalfa stomata opened in the mean- 

 time and also reached maximum at this hour. At 6 p. m. the onion 

 stomata had closed to 40 per cent, remaining in this condition for an 

 hour, then to 30 per cent at 8 p.m. for an hour, finally closing to 10 per 

 cent at 10 p. m. The alfalfa stomata closed uniformly to 10 per cent 

 at 9 p. m., but showed no further closure the following hour. At 11 

 p. m. night opening had set in for both species, onion reaching 15 per 

 cent, remaining there for an hour and then closing slowly and com- 

 pletely by 2 a. m. Alfalfa stomata, on the other hand, opened gradu- 

 ally to a maximum of 50 per cent at 1 a. m. and then closed slowly 



