364 
THYSIOLOGY. 
BOOK II. 
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. 
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 confer vse and 
several aquatic herbs present the same system of discolour- 
ation. Air evidently produces its effect by altering their 
chromule, probably by abstracting its carbon ; for such is the 
ordinary effect of the air upon dead vegetable matter. Cha- 
ras, in particular, when dried in the air, become quite white ; 
this tint is no doubt to be ascribed to the alteration of their 
chromule, 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. 
The straw-coloured hue of the green parts in a great number 
of dead vegetables after death depends, on the one hand, upon 
the oxygenation of their chromule, and on the other on the 
decarbonization determined after death by the action of the 
oxygen of the air. Most leaves when they die are invested 
with a uniform 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 in leaves, as well 
as 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. 
" Not less curious nor less difficult to reduce to any in- 
telligible laws is the subject of Vegetable Odours. Our 
senses are daily gratified by the sweet perfumes exhaled by 
the leaves and flowers that surround us ; and art exhausts 
its skill to preserve them by means which enable us always to 
