\ arious attempts liave been made to determine the cause
of the remarkable circulation in Chara and Nitella. The insufficiency
for this purpose of the well-known laws of endosmosis
and exosmosis has been ingeniously shown by Mr. Slack
in a note inserted in the Second P a rt of vol. xlix. of the
“ Transactions of the Society of Arts,” tha t lamented inquirer
having ascertained that the cii'culation continued in all plants
in which it has been observed when the portion was immersed
in oil or mucilage, and in strong saline solutions, and even
Avhen not in contact Avith fluid of any sort, but surrounded
only by the dry atmosphere.
In a letter addressed to the Boyal Academy of ScicBces,
of Paris, in the sitting of November 23d, 1829, hy M. Du-
trochet, some highly interesting remarks occur relative to the
circulation of Uiara. The discovery was made very many
years ago by Count Bumford, tha t water placed In vertical
tubes circulates. In order that the circulation of so transparent
a fluid may he the better appreciated it is requisite to suspend
in it some molecular substance, the best for which purpose M.
Dutrochet has found to he a fcAv drops of milk, the specific
gravity of Avhich is nearly equal to th a t of the water. Many
other substances Avill circulate in Avater, such as very fine
saAvdust, amber reduced to powder, and rasped cork; but
these being of greater specific gravity than the element in
which they are suspended, sooner or later subside to the
bottom of the tube.
IM. de Bailif illustrated, by means of an apparatus cons
tru c ted on this principle, th e circulation which takes place
in the cells o f Chara, &c. M. D u tro ch e t, however, in the
le tte r above cited, examines this curious physical phenomenon
more closely. H e found th a t th e efficient cause o f th e circulation
Avas caloric, th a t the rap id ity o f th e motion depended
on the amount of caloric to which th e tu b e and its contents
Av^ere subject, and th a t th e direction of th e c u rre n t was governed
by the same agent, the ascending stream being placed
ahvays on the side of th e tu b e Avhich received the most heat.
M. D u tro ch e t observed, moreover, th a t lig h t exerted a poAver-
ful influence on the circulation, Avhich influence Avas a ttr ibutable
to the caloric which accompanied its rays, and also that
mineral substances in solution accelerated its speed. The
caloric and the mineral substances in solution were supposed
to exert this power by causing the greater separation of the
molecules of the water, and thus increasing their mobility.
Pressure was found to have a contrary influence, and to retard
the circulation by, it Avas presumed, occasioning the
closer approximation of the molecules, and so impeding their
mobility. Viscid organic matter, such as gum, produced the
same effect, and for a similar reason.
The causes not favourable to the sustenance of this physical
circulation, therefore, are, the absence of caloric, or cold, and
of light, upon which it follows th a t the motion ought not to
take place during the night, pressure, and the solution of
viscid substances in the liquid ; all of which act in the same
way, viz. by impeding the facility of motion between the
aqueous molecules. On the contrary, the favourable causes
are all those things Avhich act in an opposite way, by increasing
th a t facility.
A very curious fact is related by Dutrochet In reference
to the circulation of aqueous fluid in tubes. I t is this : that
if a drop or two of acid, of alkaline solution, or a solution
somewhat saline be added to the water contained in a tube,
and which has had milk mixed with it, these substances, more
weighty than water, precipitate themselves, and become dissolved.
“ This solution being achieved, the water is no
longer susceptible of presenting the circulatory movement
by means of simple diffused light ; It only presents this movement
at its superior part, and this only when the tube Is
exposed to the direct light of the sun, the continued action
of which on the tube for many hours can scarcely penetrate
the circulation to the depth of an inch in th a t water, whose
molecules have acquired a very extraordinary molecular fix ity .
I consider this fixity as the result of a regular disposition of
the molecules of the liquid. Indeed, when one agitates this
liquid endowed with molecular fixity, it becomes immediately
susceptible of circulation under the Influence of the simple