20 PERIODICAL OPENING AND CLOSING OF FLOWERS. 



of the light of the sun than the white and red, for in a like numher 

 of species of each colour there is in hoth the first groups not only 

 a far greater number of plants whose blossoms open and shut 

 periodically, but there is a far shorter period of the operation of 

 light on them, by which the plants wake from sleep, and com- 

 pletely expand their flowers, and consequently they fall the 

 sooner into a state of repose. 



Again, as regards temperature we have — 



While then the colour of the blossoms seems to have no influence 

 on the temperature at which plants wake from sleep, those which 

 have blue and yellow flowers require a higher temperature for full 

 expansion, and are capable of enduring greater heat than the white 

 and red, and consequently the co-efficient of temperature is less. 

 In general, the difference of this influence of temperature seems 

 to increase in proportion as it is raised above that degree at which 

 plants wake from sleep. 



White and red blossoms need a longer action of the light of the 

 sun, and a higher temperature of the air, in order to expand then- 

 petals, than blue and yellow ; but when the epoch has arrived at 

 which sleep ceases, this proportion is changed, and the maximum 

 expansion of the latter requires a higher temperature, and this 

 ratio continues to that time of the day when sleep again takes 

 place, since the blue and yellow fall into that condition earlier, 

 and at a higher tempei'ature, than the white and red. 



There is a very close connection between this last inquiry and 

 the question what influence insolation has on the time of sleep 

 and its limits, taking colour into consideration. The following 

 result is yielded by the tables : — 



End of sleep. 

 + 0-5 



Colour of petals. 

 White and red (after beginning or "I 



end of insolation) J 



Blue and yellow (before beginning "• ~ k 



or end of insolation) . . . j 



Beginning of sleep. 

 + 0-9 



. — 1-6 



The white and red open after the commencement of insolation, 

 and close after the insolation has ceased, while the blue and yellow 



