DAILY MOVEMENT IN ALFALFA. '2~> 



mained widely open for a somewhat longer time. Complete closure 

 occurred at approximately the same time, and hence these more 

 active stomata closed more rapidly than the others, as well as opening 

 more widely. In most sets of epiderm examined were found com- 

 plete series of stomata, from these very functional ones to others 

 which seemed entirely functionless. By keeping a leaf under the 

 microscope, held on a slide without a cover-glass, and shutting off 

 the light between periods of observation, this individual variation in 

 the action of the stomata could be followed in detail. Naturally, 

 the behavior of the stomata of a leaf treated in this manner is not 

 like the behavior of the stomata on the other leaves of the plant. 

 Still it may be safely assumed that when there is a more or less indi- 

 vidual reaction by each stoma to rather unnatural conditions, there 

 is also similar variation under more natural conditions. This is the 

 only assumption that will explain the differences in the degree of 

 opening found in the various stomata of a strip or at the moment 

 of observation in a living leaf. However, most of the stomata act 

 alike; at least 80 of each 100 in alfalfa are alike within the accuracy 

 of measurement, and the greater part of the remaining 20 are suffi- 

 ciently near the average, so that they have no effect. Approxi- 

 mately 2 per cent of all the stomata in an alfalfa leaf are function- 

 less, because of incomplete development or a more obscure cause, and 

 3 per cent are superfunctional, opening often to nearly twice the 

 normal maximum and usually changing the degree of opening with 

 remarkable speed. These erratic ones counterbalance each other, and 

 have no real effect upon the opening found in a leaf, except during 

 a period of stress. In cereals, however, this variation is important 

 at all times. 



From June 8 to September 1, 1916, 16 additional series were 

 made, each continuing over a period of 24 hours or more, while 1 

 was 68 hours in length; 12 of these series contained a set of alfalfa 

 strips, largely as a basis for comparison with the stomata of other 

 plants. Figure 9 shows the curves of the stomatal movement in the 

 upper epiderm of alfalfa, plotted so as not to intersect each other. 

 In addition to the 5 already given, 6 have been selected to show the 

 progressive changes in movement as the soil became drier, the weather 

 hotter, and the evaporating power of the air increased. The last two 

 curves are of special interest. Edith B. Shreve (1916: 114) states: 



"Under typical conditions the stomata [of Opuntia versicolor] begin to close 

 soon after sunrise and they appear to be completely closed by noon. They 

 begin to open at 5 to 6 o'clock in the evening and continue to open through- 

 out the night, the maximum size occurring between 3 and 6 a. m." 



This statement applies almost word for word to the behavior of 

 alfalfa stomata under conditions of low water-content, high temper- 

 ture, continuous brilliant sunshine, and very dry air; in other words, 



