MOVEMENT IN PLANTS. 257 



for a distinct closing movement ; this may be done by 

 marking two opposite ray-flowers with a spot of ink and 

 measuring the horizontal distance between the tips of these 

 flowers before and after placing the plant in darkness. 



364. Effect of continued Darkness. Keep some 

 Daisies in darkness for several days, using either dug-up 

 plants in pots, or cut flower-heads ; the darkening may be 

 effected by covering some Daisies with an inverted flower- 

 pot with the hole plugged. After two days, the heads open 

 again, though not fully, and then remain in this condition, 

 showing that the alternation of light and darkness is neces- 

 sary for the continuance of sleep movement. In the case 

 of G-oatsbeard, the inflorescence opens again after being 

 kept about nine hours in darkness. 



365. Effects of Temperature and Light Changes. 



The flower-heads of Daisy are sensitive to temperature as 

 well as to light, but their responses to temperature are 

 feeble as compared with those made by Tulip and Crocus 

 flowers. If closed Daisy _heads are brought indoors at 

 night, they do not open, though the rise in temperature 

 may be as much as 15 C. ; nor does a corresponding fall 

 in temperature make the open head close during the day. 

 But if in the morning the closed heads are warmed through 

 15 they will open, and if at evening the open heads are 

 cooled through 15 they will close. 



366. Sleep Movements of Non-pulvinate Leaves. 



In many plants the young growing leaves perform sleep 

 movements, but these become less and less marked as the 

 leaf grows older. In other cases the fully grown leaves 

 retain the power of performing sleep movements, and these 

 leaves are distinguished by having a pulvinus or motile 

 organ. 



In the movements of non-pulvinate leaves, the day position is 

 more or less horizontal, and the night position vertical, the move- 

 ments being due to curvature of either the petiole or the base of the 

 blade. The leaves may sink at night, e.g. Balsam, Hop, Polygonum 

 convolvulus ; or they may rise and stand erect, e.g. Chenopodium, 

 Polygonum aviculare, Stellaria, Linum, Mirabilis. In both cases 

 P.B. 17 



