FUNCTIONS OF BRACTS AND FLORAL ENVELOPES. 827 
young and tender parts placed within them from injury. When 
green, as is commonly the case with the bracts and sepals, their 
colour is due to the presence of chlorophyll in their component 
cells, and they then perform the same functions as ordinary 
leaves. But when of other colours than green, as is usual with 
the petals, and occasionally with the bracts and sepals. they 
appear to have, in conjunction with the thalamus, a special func- 
tion to perform ; which consists in the production of a saccha- 
rine substance from the amylaceous matter stored up in them. 
This saccharine matter is designed more especially for the 
nourishment of the essential organs of reproduction. That such 
is the function of these parts seems to be proved by the varying 
composition of the thalamus at different periods of the flowering 
stage. Thus, at the period of the opening of the flower, the 
thalamus is dry and its cells are filled with amylaceous matters ; 
as flowering proceeds, these matters become converted into sac- 
charine substances ; and, finally, after flowering, the thalamus 
dries up. In fact a similar change takes place in the process of 
flowering to that which occurs in germination, where the amy- 
laceous matters are in like manner converted into those of a 
saccharine nature. When the saccharine matter is in excess, 
during the process of flowering, it is found upon the parts in a 
liquid state, and may be removed without the flower suffering, 
indeed one of its chief uses seems to be that of determining the 
direction of the entrance of insects into flowers which receive 
entomophilous fertilisation. 
During this conversion of amylaceous into saccharine matters, 
oxygen is absorbed in great quantities from the atmosphere, and 
carbon dioxide given off in a corresponding degree. Hence, the 
action of the parts of the flower which are of other colours than 
green upon the surrounding air under the influence of solar 
light, differs from that of the leaves and other green organs. 
The absorption of oxygen takes place in a still greater degree in 
the essential organs of reproduction ; hence, such an effect is 
more evident in hermaphrodite flowers, than in those in which the 
stamens and carpels have been more or less changed into petals 
—that is, when the flowers have become partially or wholly 
double. It has been proved, also, that staminate flowers absorb 
more oxygen than pistillate ones. 
The combination which under the above circumstances takes 
place between the carbon of the flower and the oxygen of the air, 
is also attended by an evolution of heat, which indeed is always 
the case where active chemical combination is going on. This 
evolution of heat in the majority of flowers is not observable, 
because it is immediately carried off by the surrounding air ; 
but in those plants where many flowers are crowded together, and 
more especially when they are surrounded by sucha leafy struc- 
ture as a spathe, which contines the evolved heat, it may be 
readily noticed. The flowers of the male cone of Cycas circi- 
