354 The Philippine Journal of Science \m 



These facts led us to undertake an experiment to determine 

 what changes occur in the length of stems of Cestrum nocturnum 

 during the day and the night. The young shoots of this plant 

 appear to respond very quickly by curvature to changes in their 

 water content. When fully turgid at night the stems bend 

 upward slightly. Early in the day they become less stiff and 

 assume a straight position. This on dry days is followed by 

 a downward bending until they hang down in a markedly wilted 

 condition. Later in the day, when they are no longer exposed 

 to direct sunlight, they straighten and either late in the after- 

 noon or early at night again become slightly bent upward. 

 These various changes in curvature may probably be taken to 

 indicate the presence of different amounts of water in the shoots. 



The two plants used in the experiment were in a yard in Ma- 

 nila and were between two houses; owing to their position the 

 time of exposure to direct sunlight was different for the two. 

 In order to measure the changes in the length of a shoot, an 

 ink line was placed as near as could conveniently be done to 

 the growing apex, and another farther back, in a region where 

 elongation had apparently ceased. The distance between these 

 two lines was then measured at intervals during the day and 

 night. 



On one plant four shoots were selected for such measurement. 

 A Livingston white spherical atmometer was placed near these 

 four shoots, in such position that its exposure to direct sunlight 

 was practically the same as was that of the four shoots. A 

 thermometer was placed in the shade near the shoots and read 

 at the same time that the measurements were made. Measure- 

 ments were begun at 12.30 in the afternoon, April 29, 1918, and 

 continued until 8.00 in the evening, the following day. The 

 results of these measurements, together with observations on 

 environmental conditions and on the appearance of the shoots, 

 are given in Table I. The evaporation, reduced to Livingston's 

 standard, is given in cubic centimeters per hour. The period 

 covered by the evaporation reading for any particular hour 

 begins at the time of the preceding measurement. 



When the first measurements of length were made all of the 

 shoots were in the shade, and they continued so until dark. At 

 8.00 in the morning of the following day they were exposed to 

 direct sunlight, and continued to be so exposed until after 11.00 

 in the morning, when the sky became overcast with clouds. Dur- 

 ing the afternoon the sun again shone brightly, but by this time 

 the plants were in the shade of a building. At 12.30 in the after- 



