94 The Philippine Journal of Science 1923 



of an ordinary flat leaf; but some wilting generally occurs in 

 the day, and the two wings swing downward, about the midrib 

 as an axis, so that their lower faces approach each other as 

 wilting begins and progresses. These changes in leaf position 

 are principally due to alterations in the turgidity of the cells 

 in a double "hinge," composed of two narrow, colorless strips 

 of tissue visible on the lower surface of the leaf, one along each 

 side of the midrib for its entire length. By the action of the 

 hinge, the two wings assume various positions, swinging upward 

 or downward as if hinged to the midrib. The magnitude of the 

 angle between the two wings may be approximated by deter- 

 mining the distance between the two free edges of the leaf ; this 

 distance may be called the apparent leaf width. 5 The apparent 

 leaf width would become zero if the leaf were completely closed, 

 which does not occur, however, in the case of banana leaves. 

 Fluctuations in the apparent leaf width are probably approxi- 

 mate indices of corresponding fluctuations in the turgidity of 

 the hinge cells, and these changes are probably primarily deter- 

 mined by the water content of these cells. The leaf movements 

 may therefore be considered as indices of changes in foliar 

 water content. 



A study was made of the angular changes in the leaf wings, 

 using forty-six selected leaves exposed to full sunlight, the 

 apparent width of each leaf being measured every two hours, 

 from 8 a. m. to 6 p. m., May 17. An equal number of similar 

 leaves that were partly shaded during the day were also 

 measured in the same manner. The averages of the values 

 secured are shown in Table 6. The actual averages are given 

 and also the corresponding relative values, the latter being 

 expressed, for each series, as percentages of the actual value 

 for 8 a. m. 



An examination of the data of Table 6 shows that in each 

 set the apparent width of the leaves decreased from 8 a. m. 

 until noon, and then increased until 6 p. m. This may be taken 

 to mean that the turgidity, and presumably the water content, 

 of the hmge cells decreased until about noon and then increased 

 m the afternoon. The reversible movements of the wings of 

 these leaves are apparently due to alterations in the moisture 

 content of the thin-walled hinge cells that lie in a row at each 



'The trigonometrical reasoning on which this statement is based has 

 been presented in another paper. See Trelease, Sam F, Incipient dry- 



ZTm^Ztet™ by movements of coconut ™ Am - Journ - 



