DAILY MOVEMENTS OF LEGUMINOUS LEAFLETS 43 



begins to go down in the west. The xerofotic response is par- 

 ticularly noticeable during the hot or dry season, since the sun 

 is much in evidence during that time. In a rapid assumption 

 of the xerofotic position a certain amount of upward curling is 

 noted at the tip of the leaflet. This upward curling is a form 

 of the generalized type of xerofotic response in contradistinction 

 to the localized type, in which the difference in turgidity is 

 localized in the pulvinus. 



In giving a daily program of the movements of leguminous 

 leaflets, two classes must be considered according to the night 

 position of the photeolic movement. In Gliricidia sepium, as an 

 example of plants with, a drooping night position, we have the 

 following changes of position during the 24 hours. From the 

 drooping night position of the leaflets we have the gradual change 

 to the horizontal position as the sun rises. The beginning of 

 the change generally takes place before the sun is actually above 

 the horizon, but the completion is not effected prior to sunrise. 

 On cloudy days this movement takes place somewhat later in 

 the morning. Dark chamber experiments have shown that this 

 change is a weak habit and will persist for a couple of days in 

 the absence of the usual stimuli. On cold mornings — that is, 

 with the temperature below 70° F. — the movement is noticeably 

 delayed. 



With the assumption of the horizontal position the leaf is in 

 the ''day" position. This should normally be maintained until 

 a little before night. However, if the sun is shining strongly 

 on the leaf, another movement will take place. This is the 

 xerofotic movement. The cells of the upper side, exposed to 

 the direct sun, lose more water and are consequently less turgid 

 than those of the under, less exposed side. If for any reason, 

 the direct sun is cut off from the leaf the horizontal position 

 is resumed. If, however, the sun remains strong throughout the 

 day, the xerofotic position is maintained until the effects of the 

 sun are lessened by its sinking in the west. Thereupon the 

 horizontal position is assumed and maintained until the condi- 

 tions for the photeolic movement are brought about. In the 

 group, of which Gliricidia is a member, this consists of a move- 

 ment of the leaflet from the horizontal position to the drooping 



THE PLANT WORLD, VOL. 19, NO. 2, 1916 



