436 
PHYSIOLOGY. 
BOOK II. 
changes that white flowers undergo in a few hours offer similar 
evidence. Cheiranthus chamaeleo has a flower at first of a 
whitish colour, which afterwards becomes lemon-yellow, then 
red, and slightly violet. Stylidium fruticosum has its young 
petals pale yellow, its old ones white tinged with red. The 
flowers of QEnothera tetraptera are at first whitish, afterwards 
pink, and finally red. The petals of the common Tamarind are 
said to be white the first day, and yellow the second. The 
corolla of Cobaea scan dens is greenish-white the first day, and 
violet the' next. Finally, Hibiscus mutabilis unfolds its blos- 
soms 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 observed that, on the 1 9th 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 l9th of October was remarkable for 
the centigrade thermometer not rising higher than 19°, while 
the ordinary temperature of the flowering season of the Hi- 
biscus is 30° centigr. ; so that it would seem that heat has 
some important connection with the developement of colour; 
and this notion is in accordance with the fact, that white 
flowers are most common in cold countries. Korthals sup- 
poses he has ascertained experimentally that the change in 
the Hibiscus flowers from white to red is owing to oxygen- 
ation. i^An. Sc,, n. s., ix. 63.) 
All the brilliant spectacle of vegetable colours tends to 
disappear either in consequence of accidents or upon the ap- 
proach of death ; and what renders this subject the more 
curious is, 1. that discolouration is often determined by the 
same agents as in other cases produce colour ; and 2. that 
certain organs which have no colour while alive gain when 
dead a very decided tint. 
Solar light seems to be the most usual cause of those losses 
or changes of colours. While plants are alive, it acts, as we 
have so frequently seen, by colouring them ; but in certain 
cases its too powerful action discolours them. Thus the cul- 
tivators of tulips place their flowers under a tent, knowing 
very well that the direct action of the sun tends to alter their 
colours more promptly than would be the case in the shade. 
