chap. XVI. 
COLOUR. 
437 
A great number of delicate flowers, particularly of those 
belonging to the cyanic series, exhibit this phenomenon. 
Most aquatic plants gain in death a whitish hue ; this is 
particularly remarked in sea-weeds, which, from the most 
brilliant blue or green, pass to white when they die, an effect 
which seems to be augmented when they are exposed to air 
and light ; but the exact mode of action of these several 
agents has not been appreciated. Fresh- water Confervaeand 
several aquatic herbs present the same system of discolour- 
ation. Air evidently produces its effect by altering their 
chlorophyll, probably by abstracting its carbon ; for such is 
the ordinary effect of the air upon dead vegetable matter. 
Charas, in particular, when dried in the air, become quite 
white; this tint is no doubt to be ascribed to the alteration 
of their chlorophyll, but in all probability also to the enormous 
quantity of calcareous matter that those plants, while alive, 
fix in their tissue ; other cases of a like nature may be easily 
named. Most leaves when they die are invested with a uni- 
form russet colour ; it has some analogy with what happens 
in bletted fruits, such as the medlar. Such a state of the 
leaf may very well be owing, both in leaves and in fruits, to 
an alteration in their principles, analogous to putrefaction or 
fermentation. It is always accompanied with a great loss of 
water; but we have no direct evidence as to the nature of 
this change. 
F F 3 
