12 BEHAVIOR OF STOMATA. 



forceps projecting about an inch beyond the tip of the scalpel blade. 

 The cut once made, the instrument was twirled half a turn to bring 

 the forceps into position, the cut end seized, and the epiderm plunged 

 into the previously opened vial of alcohol well within a second. A 

 good flash-light was usually used for night work, or at times an elec- 

 tric light from a field socket nearby. This light was sufficient to 

 select the leaves desired and to strip them, the process taking about 

 a minute, an interval too short to cause opening by the light used. 



A number of precautions must be observed in stripping if the 

 results are to be reliable. Stomata vary greatly in sensitiveness in re- 

 sponse to stripping. Those of cereals and grasses generally are very 

 sensitive to exposure to air and must be plunged into the alcohol in 

 less than a second, while those of potato or Rumex patientia may be 

 kept out for a much longer time. A strip of this latter was kept ex- 

 posed to the air for a full minute in one experiment without change 

 of stomatal opening. Corn is not so sensitive as wheat or barley to 

 such exposure, but even with this plant it is best to reduce the process 

 to less than 2 seconds when stripping. Another precaution is not to 

 strip a wet leaf. If rain or heavy dew has fallen, the moisture should 

 be removed with filter-paper or absorbent cotton, since the water 

 carried with the strip may dilute the alcohol seriously. The water 

 on the strip may also affect the stomata before the strip reaches the 

 alcohol, especially when, as is usually the case, capillarity causes the 

 water to be pulled to the under side of the strip. This latter objec- 

 tion depends a good deal upon the kind of plant stripped. The epi- 

 derm of alfalfa, which has sensitive stomata, has been kept in water 

 for an hour or more after stripping and no appreciable difference found 

 as compared with a similar strip plunged into alcohol in the usual 

 manner. However, even with alfalfa, the water carried with the 

 strip will often dilute the alcohol in contact with the strip long 

 enough to cause partial or complete collapse of the guard-cells. 



Great variation in the size of stomatal apertures on various leaves 

 may be found at any one time, not only on leaves of different indi- 

 viduals, but also those of the same plant. They even vary much 

 at times on the same leaf. The causes for this are many. No two 

 plants are found in exactly similar situations, nor are the leaves on 

 the same plant under identical conditions. The usual cause for 

 difference in stomatal openings in the various leaves on the same 

 plant lies in their age or, more properly, their degree of maturity. 

 A leaf does not have functional stomata until it is fully grown, and 

 young stomata do not open so soon or stay open so long as the 

 stomata on the more mature leaves. The same is true in even greater 

 measure of an aging leaf. Long before an old leaf turns yellow and 

 falls, the stomata have ceased to function and remain permanently 



