PERIODICAL OPENING AND CLOSING OF FLOWERS. 21 



open and close before the beginning or end of insolation, whence 

 we have a fresh confirmation of the position that white and red 

 blossoms possess a weaker sensibility for light than the blue and 

 yellow. 



It remains under this head to consider the influence of colour 

 on the magnitude of the phase. The tables give the following 

 mean result : — 



We see here that the difference of phase in either group is the 

 greatest at the period of the minimum, that it decreases as the 

 mean phase approaches, aud has nearly vanished at the time of 

 the maximum ; the daily change, too, is greater in the blue and 

 yellow than in the white and red. 



5. Dependence on the natural orders of plants. — The only remain- 

 ing point under consideration is the dependence of the phenomena 

 of sleep in plants on the families to which the plants belong. It 

 appears then, from the author's observations, that the duration of 

 sleep, or the time of its commencement and cessation, is as little 

 dependent on the order in which they come in the natural system 

 as the epoch at which they are in full splendour. After an 

 examination of the phenomena exhibited, it appears generally 

 that the 



Period of waking . , 



Period of full expansion = „ 



Period of commence- 1 



ment of sleep . . j 



= from 3 a.m. 

 = „ 10 „ 



9 a.m. =: 6 hours 

 2 p.m. — 4 



3 p.m. — 7 



= 4 



These periods are limited with tolerable precision, so that very 

 rarely a family of plants goes through the different phases beyond 

 the prescribed limits. We are, therefore, compelled to allow that, 

 however manifold and different the causes may be which modify 

 the daily range of phase, still the principal cause of periodicity is 

 the same in every family, and this is the diurnal range of insola- 

 tion ; when it begins, sleep ceases, full expansion is synchronous 

 with its maximum, and sleep again takes place when it ceases. 

 The relation between the magnitude of the phase and the series 

 of natural families is as unimportant as that which regards the 

 limits of sleep. 



