COLORS OF FILTERS. 



453 



of surface : e. g., in the case of velvety petals the color is often 

 softened, sometimes to a remarkable extent. 



1166. Contrasted colors are often seen in a single flower. In 

 general these are so disposed in spots or lines as to suggest that 

 they bear a direct relation to the point where the nectar is se- 

 creted ; hence such color-marks were called by Sprengel nectar- 

 spots or nectar -guides. But in some cases flowers have conspicu- 

 ous spots without being nectariferous ; e. g. certain poppies. 



1167. Darwin cites the following case as showing that nectar- 

 marks have been developed in connection with the nectaries : 

 '* The two upper petals of the common Pelargonium are thus 

 marked near their bases, and I have repeatedly observed that 

 when the flowers vary so as to become peloric, or regular, 

 thev lose their nectaries and at the same time the dark marks. 



J 



When the nectary is only partially aborted, only one of the 

 upper petals loses its mark. Therefore the nectary and these 

 marks stand in some sort of close relation to one another, and 

 the simplest view is that they were developed together for a 

 special purpose ; the only conceivable one being that the marks 

 serve as a guide to the nectary." l 



1168. The colors of the flowers in certain species change more 

 or less after opening ; thus many Borraginacese turn from red 

 to blue even during a short space of time. One of the most 

 interesting cases of this change of color is presented by Arnebia. 

 AVhen the flower opens each lobe of the yellow corolla is con- 

 spicuously marked by a deep purple spot ; after a few hours this 

 begins to fade, and by the next day entirely vanishes. 



1169. Of all colors of flowers white, pale yellow, and yellow 2 

 are the most common. 



1 Effects of Cross and Self Fertilization, 1876, p. 373. 



2 The following table by Kohler and Schiibeler (cited by Balfour) exhibits 

 the relative frequency of certain colors in the plants of twenty-seven different 

 families of plants : - 



