THE PHYSIOLOGY OF STOMATA. 



measurements given are maximum and minimum in eyepiece parts (r part = 

 3.5 micra) and are correct within an error of 0.25 part, and the averages 

 deduced are very nearly correct. For darkening the parts to be studied, I 

 used covered tin pails, sheathed with white cloth which could be kept wet, so 

 keeping the temperature nearly normal. 



June 21, 1906. Ampelopsis quinquefolia. Leaves attached to plant darkened in evening 

 and illuminated for whole period indicated, forenoon (9 h 3o m to n h 2o m ) and afternoon, 

 (ih^o to 3 h 30 m ). A different leaf was used for morning and afternoon. Plants on 

 eastern exposure normally receive sunlight till n a. m., while those on the west are 

 shaded till that hour. Measurements given in parts (i part = 3.5 micra). 



TABLE 43. Stomatal pore dimensions. (Fig. 28.) 



The measurements show that the average initial opening in the afternoon 

 period of illumination was about one-half that of the morning period. Rela- 

 tively to the initial opening, the greatest opening attained (in i hour) was the 

 same. The ratios between the initial and maximum openings for the morning 

 hours are I (shade), 2; IV (sun), 3. For the afternoon hour they are III 

 (shade), 3; II (sun), 2. 



The total average amount of movement was, however, greater in the morn- 

 ing, although case I (a. m.) and case II (p. m.) showed the same amount. 

 It seems possible, from a mechanical point of view, that it would take longer 

 to effect the same amount of opening when the stoma is closed, or nearly so, 

 than when it is already well started in opening. It is usually easier to cause 

 closing in a wide open than in a slightly open stoma. The amount of move- 

 ment is, of course, a measure of internal forces, and these are the expression 

 of protoplasmic activity. If this mechanical consideration is the fact, then 

 we have no evidence that induced rhythm is present, the opposite of which 

 would seem to be true at first glance, and which must be admitted as the 

 possibly proper view and in agreement with Francis Darwin's interpreta- 

 tion of his experiments. 



