THE STEM AND THE LEAF 



65 



In a great number of trees the young leaves from recently 

 opened buds stand erect or hang straight down. In one 

 tropical species 1 it is estimated that these young drooping 

 leaves do not get more than -5^ as intense illumination as is 

 received by the most exposed of the mature 

 leaves. 



59. Daily movements of leaves. It is com- 

 mon to find leaves assuming different posi- 

 tions during different portions of the day, or 

 even whenever (as from the long continuance 

 of clouds over the sky) the intensity of the 

 sunlight is much altered. These daily changes 

 of position are particularly frequent in plants 

 of the Pea family, and many of these have a 

 special cushion-like organ, the pulvinus, at 

 the base of the leafstalks or of the leaflets, 

 which produces the movements. Sometimes, 

 as in Fig. 52, there are only two principal 

 positions assumed during the entire twenty- 

 four hours, one for the day, the other for the 

 night. In other cases there are at least three 

 well-defined positions, as in the case of the 

 black locust leaf. In this the leaflets droop 

 at night, remain nearly horizontal in ordinary 

 daylight, and stand erect in full sunlight. 



It is certain that the plant gains some ad- 

 vantages from the change from horizontally 

 placed to vertically placed leaflets, and the 

 reverse. The horizontal position is (as already 

 stated) favorable for photosynthesis in mod- 

 erate light, and the vertical position hinders undue absorp- 

 tion of intense sunlight by the chloroplasts. What benefit 

 the plant gets from the assumption of the night position by 

 the leaves, and of how much importance this is, are questions 

 as yet unsettled. 



1 Amherstia nobilis, from Burma. 



FIG. 53. The purple 

 wood sorrel, with 

 the leaves in the 

 nocturnal position 



One third natural 

 size 



