No. 423.] NORTHERN POLYPETALOUS FLOWERS. 239 
4. The changes of color and their sequence in individual 
flowers are noteworthy. Green changes to white (Cornus), to 
yellow. (Thlaspi, Cardamine), to red (Hydrangea), to purple 
(Clematis), to violet (Cobzea) ; white changes to green (sepals of 
Helleborus niger), to yellow (Lantana), to red (Dianthus, Hibis- 
cus mutabilis), to blue (many large blue flowers remain white 
until nearly ready to expand) ; yellow changes to white (Draba), 
to red (/Esculus) to blue (Myosotis); red changes to blue 
(Venetus and many Boraginacez) ; violet and blue may turn 
purplé, green, or white in fading. The tendency of green, 
white, and yellow to change to red or blue is much stronger 
than the reverse. 
5. The floral colors are often correlated with the colors of 
the stems and leaves, as in Sedum. The foliage of the plants 
with white flowers is, as a rule, paler than when the flowers con- 
tain pigments. The development of bright colors in autumn 
leaves presents a series of color changes, which are in part par- 
allel to those which occur in flowers. With the disappearance 
of the chlorophyll the leaves become whitish, yellow, or red, 
according as the cells contain no pigment, or solid yellow gran- 
ules, or red pigment dissolved in the cell sap. The leaves of 
many plants are yellowish green, due to the presence of a yellow 
pigment. Green, yellowish-green, and greenish-yellow flowers 
contain chlorophyll, and though usually small are occasionally of 
large size. Many white and yellow flowers are derived directly 
from the primitive green. White is usually a structural or opti- 
cal color due to the unequal reflection and refraction of light by 
the intercellular air spaces and the cells devoid of pigment. 
White flowers are a less tax upon the energies of the plant. 
Flowers of all colors may revert to white, which is commonest 
in nature and most true to name under cultivation. If with the 
disappearance of the chlorophyll there is an insoluble yellow 
pigment in the cells, the petals are a pale yellow, and with its 
increase change to bright yellow or orange. The development 
of anthocyanin, or red pigment, dissolved in the cell sap, changes 
white flowers to red and yellow flowers to scarlet. With a 
decrease in the acidity of the cell sap the red flowers become 
blue. Müller's observations led him to the conclusion that the 
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