INTRODUCTION. 13 



closed. With experience one learns which leaves are mature and 

 actively functional, and these alone should be used in a series. 

 Another cause of variation in the leaves on a plant is due to the fact 

 that the stomata of leaves that are shaded are often closed when 

 those of unshaded leaves are open, or open when the ones on the 

 upper leaves are closed. But when enough stomata are measured 

 to insure checking out the extremes, the degree of opening in the 

 mature leaves of the same plant exposed about equally to the light 

 agree very well. Usually three leaves of each set of plants were 

 stripped each hour, giving three strips of epiderm for each leaf- 

 surface for each hour. This insured against loss due to inadvertently 

 stripping an immature or aged leaf. Such an occurrence was very 

 rare after some experience had been gained in selecting the proper 

 leaves. It also served as a check upon similarity of stomatal behavior 

 in mature leaves. The causes of variations of the stomatal opening 

 of different plants will be dealt with later. 



Twenty-four hour series were made of more than 60 species of 

 plants, including crop, garden, and forage plants, weeds, native 

 herbs, trees, and shrubs. The more important of these were checked 

 in the same manner as alfalfa, in order to determine the reliability 

 of the stripping method. A great many were used in but one series, 

 which was made, however, at the same time as an alfalfa series, in 

 order to determine the daily march of its stomatal movement in 

 relation to a plant whose reaction to the environmental factors was 

 more or less known. Except in the case of leaves with but one 

 stomatal surface, each series consisted of two sets of epiderm, one 

 for each surface. 



Few investigators seem to have made long, continuous series. 

 The discovery that the variation in the march of stomatal movement 

 from day to day was considerable made it evident that only a long, 

 continuous series would be of any real value. A group of short series 

 covering the entire period of the day and night, but not made within 

 the same 24-hour period, fails to reveal the actual sequence, but makes 

 an artificial one having slight relationship to the stomatal movement 

 upon any such day. Because of this, the usual series was of 24 hours' 

 duration. A number of shorter ones were made for special purposes, 

 especially for what may be termed a reconnaissance of the field and 

 for investigations on the effect of certain factors. Longer series were 

 also made quite frequently, the longest being continuous for 68 hours, 

 but the strain placed upon the investigator by so long a series is not 

 justified by the results obtained. Even a 24-hour series called for at 

 least 36 hours of continuous labor, as it required many hours of work 

 to get everything in readiness, and several hours after the series was 

 finished to complete the work. 



