162 THE PLANT WORLD, 



at Washington has the ray flowers canary yellow tipped with white. 

 Dr. Palmer says that in its wild state in Durango, Mexico, the flowers 

 assume every conceivable shade of yellow and orange and that the in- 

 tergradations and mingling of these colors is truly marvelous. This 

 marked tendency to color variation would doubtless make it easy for 

 the plant breeder to develop well marked varieties, though the natural 

 beauty of the wild plants leaves little to be desired. 



In its natural habitat Bidens heterophyUa selects moist bottom 

 lands and fields, and is able to thrive in a soil containing considerable 

 alkali. The strong healthy growth of the plant on the Potomac Flats 

 at Washington shows that it readily adapts itself to changed conditions. 

 The time of flowering is rather later than Cosmos,' this and the fact 

 that it supplies the various shades of yellow lacking in Cosmos should 

 certainly recommend it to admirers of that flower. Its only possible 

 competitor in this field is Cosmos sxdjplmreus^ a species that is very 

 little cultivated. 



The leaves of this species of Bidens are used to some extent in 

 Mexico as a substitute for tea, and it is cultivated about San Luis 

 Potosi for that purpose. The beverage is prepared in the same manner 

 as (n'dinary tea and is believed by the Mexicans to be slightly stimulat- 

 ing. When properly prepared it is not unpalatable. 



In making photographs of the fiowers of this plant a peculiar 

 phenomenon was observed. The difierence in color between the base 

 and tip of the rays is barely perceptible to the eye but on the photo- 

 graphic plate the contrast is most striking, the bases taking black, 

 much darker than the dark green of the leaves, while the ti[)s appeared 

 so dense as to print pure white, as shown in the illustrations. 



The only possible explanation that suggests itself is that the base 

 has a pure spectrum color of a very non-actinic character, while the 

 color of the tip may be a mixture of a lower spectrum yellow with 

 highly actinic blue or violet in proportions which approximate to our 

 eyes the color of the l)ase. This ex])lanation is supported by the fact 

 that when viewed through a blue glass the tips appear nearly white, 

 and this also renders it impossible that the actinic character of the tips 

 is due to the emission of ultra-violet light. 



As might be expected if the above explanation is correct the con- 

 trast is somewhat reduced when an orthochromatic i)late is used and 

 with the correct color screen in entirely disappears. 



U. S. Dept. of Agrioiilturt', Washin^lon, I). ('. 



