Vesicles of the Utricularie. 897 
and exhalation take place through the whole surface of the epi- 
blema. Experiments made by MM. Cloéz and Gratiolet have 
demonstrated that the decomposition of carbonic acid by the 
green parts of submerged plants is only effected under the in- 
fluence of light. In the dark no carbonic acid is produced, con- 
trary to what occurs in aérial plants. A certain temperature is 
necessary for the production of the phenomenon. When the 
temperature is rising, it does not commence below 15°C. 
(=59° F.); when the temperature is descending, it may con- 
tinue at 10°C. (=50° F.). Besides oxygen, the gas produced 
by the plant contains a certain quantity of nitrogen. 
If we apply the preceding facts to the leaves of the Utricu- 
larie, we find them immersed in a water which is usually very 
rich in carbonic acid; this gas is absorbed by the leaves, and, 
under the influence of light, oxygen and a little nitrogen are 
evolved. In the segments of the leaves the gases occur in the 
aériferous canals which traverse those segments; they are set 
free at different points in the form of little bubbles. We have 
not seen these bubbles issuing through the walls of the utricles, 
which appear to oppose a certain amount of resistance to their 
escape, and this may probably exert some action upon their own 
expansion. The utricles floating freely in the water become the 
seat of phenomena of endosmose and of chemical actions. This 
work goes on especially when the circumambient water presents 
a temperature of 10°-15° C. (=50°-59° F.). At first the utri- 
cles contain a mucilaginous liquid ; soon a bubble of gas is seen 
to make its appearance in this liquid, and to increase in volume: 
this is oxygen, evolved under the influence of hight and heat 
which have penetrated through the water to the utricles. The 
plant escapes from the mud and rises towards the surface; the 
sécretion of gas becomes more abundant, and the flower-stalk is 
supported above the level of the water. The oxygen secreted in 
the utricles seems to exert a chemical action, in virtue of which 
the contents of the cells of the walls undergo a transformation 
and a change of colour, becoming rose, lilac, and blue. The 
envelope thus ccloured reacts in its turn upon the interior work 
of the utricles. We know that, in fact, in organs coloured 
otherwise than green (for example, in the petals of the corolla), 
there is no longer any evolution of oxygen, but, on the contrary, 
absorption of oxygen and evolution of carbonic acid; the latter 
does not escape from the utricle, but is probably assimilated ; 
the utricle becomes filled again either with a mucilaginous mat- 
ter or with water absorbed by endosmose, and the plant increases 
in weight and descends again to the bettom of the water*. 
* In the dark, for example in the utricles which are of a black-blue 
colour, carbonic acid is no longer formed. 
