MOVEMENT IN PLANTS. 259 



369. Experiments with Wood Sorrel. Note that by 

 day the three leaflets spread out horizontally, as in Clover; 

 at night they droop so that their midribs touch the leaf- 

 stalk, while each leaflet becomes folded along the middle. 



(a) Repeat the experiments given for Clover. 



(fc) The leaves of Wood Sorrel respond to mechanical 

 stimulation, though not nearly so sensitive as Mimosa ; 

 shake a plant with its leaves in day position, and note that 

 the leaflets droop, though repeated or prolonged shaking 

 may be required. 



(c) Rub the lower surface of a pulvinus ; as compared 

 with Mimosa, the Oxalis leaflets take a long time (often 

 about an hour) to recover, and the stimulus is apparently 

 not transmitted from one leaflet to another. 



(cZ) Try the effect of striking a leaflet ; repeated blows 

 are usually required to cause drooping. 



370. General Experiments with Phaseolus. Note 

 that in Phaseolus, in the evening the petiole rises, while the 

 leaflets move downwards ; in the morning the petiole sinks, 

 and the leaflets rise and become nearly horizontal, though 

 in direct sunlight they usually droop to some extent. 



(a) Place a plant in darkness, and note that periodic 

 " after-effect " movements occur for several days ; then 

 the leaves become " darkness-rigid " and are expanded 

 horizontally. On being again exposed to normal illumina- 

 tion, phototonus is regained rapidly by the younger leaves, 

 more slowly and less completely by the older ones. 



(6) Place in darkness three pot plants of Phaseolus ; set 

 A in the normal erect position, B horizontally, C inverted. 

 Note especially that in B and C curvature and torsion 

 occur, which eventually bring the leaves into their normal 

 position. 



371. Influence of Gravitation. On the morning of a summer 

 day, take two pot plants of Phaseolus, and note the angles made 



(1) between the stalk of each of the two primary leaves and the stem, 



(2) between the blade of each primary leaf and its stalk, (3) between 

 the stalks of two or three of the trifoliate leaves and the stem, 

 (4) between the leaflets and the petiole of these trifoliate leaves. 



