336 MODIFIED CIRCUMNUTATION. CHAP. VII 



same branch, though moving a little in the same direction, also 

 l>end downwards ; they thus occupy a somewhat different posi- 

 tion, relatively to the centre of the earth and to the branch, from 

 that of the petioles on the upright branches. With respect to 

 the leaflets, they move at night towards the apex of the petiole 

 until their midribs stand nearly parallel to it ; and they then 

 lie neatly imbricated one over the other. Thus half of the upper 

 surface of each leaflet is in close contact with half of the lower 

 surface of the one next in advance ; and all the leaflets, except- 

 ing the basal ones, have the whole of their upper surfaces and 

 half of their lower surfaces well protected. Those on the oppo- 

 site sides of the same petiole do not come into close contact 

 at night, as occurs with the leaflets of so many Leguminosse, 

 but are separated by an open furrow ; nor could they exactly 

 coincide, as they stand alternately with respect to one another. 



The circumnutation of the petiole of a leaf f of an inch in 

 length, on an upright branch, was observed during 36 h., 

 and is shown in the preceding diagram (Fig. 136). On the 

 first morning, the leaf fell a little and then rose until 1 P.M , 

 and this was probably due to its being now illuminated through 

 a skylight from above; it then circumnutated on a very small 

 scale round the same spot until about 4 P.M., when the great 

 evening fall commenced. During the latter part of the night or 

 very early on the next morning the leaf rose again. On the 

 second day it fell during the morning till 1 P.M., and this no 

 doubt is its normal habit. From 1 to 4 P.M. it rose in a zigzag 

 line, and soon afterwards the great evening fall commenced. It 

 thus completed a double oscillation during the 24 h. 



The specific name given to this plant by Euiz and Pavon, indi- 

 cates that in its native arid home it is affected in some manner 

 by the dryness or dampness of the atmosphere.* In the Botanic 

 Garden at Wiirzburg, there was a plant in a pot out of doors 

 which was daily watered, and another in the open ground which 

 was never watered. After some hot and dry weather there was 

 a great difference in the state of the leaflets on these two plants ; 

 those on the unwatered plant in the open ground remaining half, 



* ' Systema Vog. Florae Peru- about its power of foretelling 



vianso et Chilensis,' torn. i. p. 95, changes in the weather-; ami it 



1798. We cannut understand the appears as if the brightness of the 



tu-count given by the authors of sky largely di termined tho open- 



the behaviour of this plant in its ing uiul closing of the leuileta. 

 native homo There is much 



