362 
PHYSIOLOGY. 
BOOK II. 
Hibiscus mutabilis unfolds its blossoms in the morning white, 
by noon they are pink, and red at night. These changes are 
constant in the West Indies ; but Ramon de la Sagra ob- 
served, that on the 19th of October, 1828, the flowers of this 
plant remained white all day in the garden at the Havannah, 
and did not become pink till noon the next day. Now this 
19th of October was remarkable for the centigrade thermome- 
ter not rising higher than 19°, while the ordinary temperature 
of the flowering season of the Hibiscus is 30^ centigr. ; so that 
it would seem that heat has some important connexion with 
the developement of colour ; and this notion is in accordance 
with the fact already mentioned, that white flowers are most 
common in cold countries. 
" Black is omitted in the two series of colours ; it appears 
to be, in all cases, an excessively dark state of brown, produced 
from a yellow base, or from a deep red ; and is of too rare an 
occurrence to be deserving much consideration, while so much 
remains to ascertain concerning commoner colours. 
" In red, there is this remarkable fact, that it belongs to 
both series ; and if the theory of oxydation should be con- 
firmed, it will appear to result from both the maximum and 
minimum of oxygenation. We may remark, indeed, that the 
various tints of red flowers differ much more from each other 
than those of any other colour. Those which become red 
through the xanthic series have usually a more brilliant 
richer, and more scarlet hue ; while such as reach it through 
the cyanic series have a decidedly violet tinge. Rose, which 
is nothing but diffused red, may belong to both series ; thus 
the rose colour of the Hydrangea evidently tends to blue, 
while that of the rose itself appears to derive its origin from 
yellow. An infusion of red flowers in alcohol takes a tinge 
more or less red ; add a little acid, and this colour imme- 
diately deepens ; sometimes, as in the Pelargonium, it passes 
to orange. Alkalies produce the most variable results in 
different plants. 
" An examination of the two most irreconcileable colours, 
yellow and blue, points out some characters which are suffi- 
ciently well marked. 
" Infusions of yellow flowers in alcohol are of a clear yellow, 
