10 PERIODICAL OPENING AND CLOSING OP FLOWERS. 



In those plants, therefore, which are fully expanded in the 

 morning, the increase of the phase does not last so long as the 

 decrease, while in those which are fully open after mid-day the 

 contrary takes place. 



When hlossoms begin to open in consequence of the cessation 

 of sleep, the change takes place generally at first slowly, then 

 more rapidly ; as it approaches its maximum, the increase is again 

 retarded ; in a few plants only the complete expansion lasts an 

 hour — more commonly not so much ; and then they begin to close 

 again at first slowly, then more rapidly, and as they approach the 

 maximum of approximation of their petals, the progress is again 

 slow, till the flower, in a more or less closed condition, remains, 

 without any regularity, many hours, till the time comes round for 

 a new cycle of phases. 



Estimating therefore the angular value of the degree of 

 expansion as follows : — 



Angular value of phase. Corresponding angle. 



Perfectly closed ... 



Half open 50 45° 



Completely expanded .100 90° 



Half reflected .... 150 135° 



Completely reflected . 200 180° 



We have — 



The maximum therefore approaches more or less 0°, and 

 in no case exceeds 45°. Anthemis cotula, Chrysanthemum cari- 

 natum, and Pyrethrum corymbosum are exceptions to this rule, 

 whose blossoms are reflected at the time of the maximum of 

 expansion. 



The mean degree of expansion not only depends on the dura- 

 tion of sleep, and is, in plants with a longer duration, smaller 

 than in those whose sleep is shorter, but is affected by the 

 magnitude of the extremes, and the daily variations of the degree 

 of expansion. Therefore the number of plants in the different 

 gradations remains nearly the same, and decreases rapidly only 

 when the phases approach the mean minimum or maximum. 



