INDUCED RHYTHM IN STOMATA. 76 



June 19, 1906. — Four species experimented on, by darkening leaves or twigs in situ by 

 'means of tin vessels; period of darkness 1.5 hours, beginning at 10 a. m. 



Three species showed no change in the measurements of the stomata — 

 a begoniaceous vine, pepper-tree, and rose. 



Ampelopsis quinquefolia showed the following results (1 part =3.5 micra) : 

 10 a. m., 0.75 to 1.5 (2) parts; n h 30 m a. m., 0.25 to 1 part; 3 p. m., 0.5 to 

 1.0, 1 to 1.75, 0.5 to 1 part; 4 p. m., o to 1.75, 1 to 2, 0.5 to 1.33 parts. 



A simple calculation shows that in the morning exposure to darkness for 1 

 hour was followed by a reduction in the average size of the stomata from 

 112 units (transverse diameter) to 62 units; that is, to not quite one-half their 

 original size. In the afternoon, however, there was but little difference. 

 According to Darwin's results, closure should be much more rapid in the 

 afternoon, and therefore my own data do not accord with his. 



June 22 and 23, 1896. — Ampelopsis quinquefolia. Plants darkened for 30 minutes in 

 morning and in afternoon; matured leaf used in each case. 1 part = 3.5 micra. 



Eastern exposure: 



June 22: 8 a. m., 2-4 (1), to 8 h 30 m a. m., 0-1; 3 h i3 m p. m., (o) 0.5-1, to 3 h 53 m p. m., 



0-0.5. 

 June 23: 6 h o8 m a.m., 0.5-1.5, to 6 h 38 m a.m., 0-1; n a.m.,* 0-0.5, (o) 0.5-1, to 

 nh-jo 1 " a. m., 0-0.5. 



June 25, 1906. — Leaves darkened. 



Ampelopsis quinquefolia: 



6 h 45 m a. m., 1-2, to 7 h 30 m a. m., (o) 0.5-1.5; i h 30 m p. m., 0-0.5, to2 h i5 m p. m., 0-0.5. 

 Hairy-leaved composite: 



yho8m a . m-) ( ) I _ 2 , to 7 h 53 m a. m., 0-1; i h 3o m p. m., 0-1 to 2 h i5 m p. m., 0-1. 

 Ricinus communis: 



7 h 5o m a. m., 0-1.5, to 8 h 3o m a. m., 0-1 ; i h 3o m p. m., 0-1.5, to 2 h i5 m p. m., 0-1. 



A plant of Iris sp. showed no difference in the morning and afternoon. 



The foregoing experiments are diagrammatically summarized in fig. 30, 

 from which, for the sake of simplicity, certain details have been omitted. 



Data derived from experiments with ocotillo give a negative answer to the 

 question. In experiments 163 to 166, as will be seen by consulting fig. 16, 

 the stomata opened more readily at n h 2o m a.m. than at 7 h 2o m a. m., by 

 3 times or more. The measurements given in the tabular summaries on 

 page 57 are quite discordant and will, therefore, not allow affirmation or 

 denial. One thing appears to be true, namely, that at about noon the amount 

 of opening movement is greater than earlier or later, and this may indeed be 

 regarded as supporting the view that an induced rhythm exists, except for 

 the objection that the stimulus was, under the circumstances, beyond control. 



The tendency of stomata to open slightly during prolonged darkness may 

 also be looked upon as affirmative evidence, in the absence of knowledge to 

 the contrary. 



* This time chosen because the direct sunlight illumination fails at 1 1 a. m. 



