20 THE CAUSES OF FLUCTTTATIONS IN TUHGESCENCE
to a si-eat degree coloured tlic various explauations wlucli haTo been advanced to accouat
for the occurrence of the sudden fluctuations in turgescence on which the movements of
masses of vegetable tissues depend. Sachs, for example, affirms that tui-geseence is essentially
due to the great resistance which protoplasm normally presents to the filtrntive escape
of any liquids which it may have endosmotically acquired, and that any sudden escape
of water from a turgescent cell necessai-ily implies a change in the nature of the protol)
lasm whereby it readily permits of filtration* He practically ascribes everything directly
to the protoplasm and nothing to the influence of the products of its activity contained
within the cell-snp, and by doing so is constrained to endow the protoplasm with certain
special properties in order to account for tlie phenomena which actually occur. In
adopting this view he entirely loses sight of the possibility that turgescence may be due to
llie osmotic properties of the cell-sap overcoming the filtrative facilities provided by the
protoplasm, and that loss of turgescence may arise as the result of diminution in the
former, and not of any increase in the latter, of these two factors; and when we come to
look for any positive evidence tending to a decision of the question, we find that, whilst
wc can readily trace the occurrence of chemical changes in the cell-sap which may well
cause changes in its osmotic jn-operties coinciding with loss in turgescence, we are
unable to find any proof of alteration in the filtrative power of the protoplasm save- the
very phenomenon which it is regarded as giving rise to.
I f we ascrihe turgescence
directly to the properties of the protoplasm, we are left to
assume that any fluctuations
in the former must be accompanied by changes in the latter;
but if we ascribe it to the
properties of the cell-sap, we can at all events show that 1
-ss in turgescence is accom-
panied b y changes in the chemical constitution of the factor.
According to the one view,
auaasthetics and all the other factors which we have just seen acting as determinants of
loss of turgescence must do so simply because they have the common property of increasing
the filtrative power of the prot.'plasm; according to tlie other, the result is due to
alterations in the properties of tho cell-sap attending depression and abohtion of respiratory
and assimilative function in the protoplasm, and of the occurrence of which we have
in many cases the clearest chemical evidence.
The conspicuous changes in the colour of common green leaves attending losses in
turgescence caused by exposure to injurious media very clearly indicate the occun-ence of
coincident chemical changes in the contents of the cells; but much more striking evidence
is forthcoming where variously coloured floral tissues form the subjects of experiment.
I n the .case of common leaves the clianges in colour accompanying depression and
abolition of functional activity (unless indnccd b y media, such as ammonia, which secure
the addition of extraneous alkaline constituents to the tissues) are such as to indicate a
rise in fixed acidity, the yellowing which takes place being precisely of tlie character
presenting itself in neutral solutions of chlorophyll on the addition of acids. This change
in certain cases may be partially ascribed to the addition of extraneous acid during the
course of the experiment; but the fact that it manifests itself in cases where chloroform or
extremes of temperature are the agents employed shows that no extraneous agency is
requii-ed to produce it. Fortunately it is not necessary to be satisfied with this chromatic
evidence alone, as it is possible to adduce positive proof that depression of functional
activity is, in some c-ases at all events, directly related to an increase in the fixed acidity
* VorlesmigiD, XVL p. 330. XSSVII. p. TÎ'S.
THE MOTOR OEGAKS OF LEAVES. 21
of the cell-sap. This is demonstrated by the following exjicriments which were originally
suggested b y a paper, published in the year 1815 in the eleventh volume of the transactions
of the Linnean Society, and to which my attention was first dii-ected by my
friend Dr. David Prain. It is entitled " O n tho Deoxidation of the Leaves of Cotyledon
calycina^' ( = Brt/opht/lhim calt/cinum), and in it the author, Dr. Heyne, points out that a
conspicuous decrease in the acidity of the tissues (which ho ascribes to processes of
deoxidation) takes place during the course of each diurnal period, alternating with a
corresponding increase in each nocturnal one.*
On testing the leaves of Ealanchoe laciniata it was at once ascertained that they
exhibited similar phenomena. Observations were first instituted during tho course of the
hot season when transpiratory loss is very active diurnally, and wiicn the amount of
fluid escaping on section of the leaves is much smaller in the evening than in the morning
; but, in spite of this, it was at once evident that the acidity, and specially the fixed
acidity of the evening (and presumably concentrated) sfip, was much lower than tijat of
the fluid escaping in the moi-ning (plate I I I . fig. 2). Systematic investigation of the
subject afforded the following r e s u l t s :—
Experiment I.—Two leaves of Kalanchoc were taken, one, a, weighing 16-93 grammes,
and the other, h, 30-75 grammes. In both of them the reaction of the cell-sap was
highly acid at 10-30 A.M. when the experiment was begun. They were both exposed to
dii-ect but broken sunshine beneath a tree, a being enclosed in a cliloroform-chamber,
and h set with the base of the petiole in water. At 3 P.JI. a weighed 12-2 grammes, was
of a pale ochreous olive colour and very flaccid; whilst b was bright green and turgid,
and weighed 31-38 grammes. The sap of a was intensely and permanently acid, that
of b was even at first only faintly acid and only contained a mere trace of permanent
acidity.
Experiment 11.—k leaf of Eahnchoe, weighing 30-3 grammes, was exposed to direct
sunshine apart from water for three hours and a quai-ter. When first set at 10-30 A.M.,
its juice was intensely acid. After exposure it weighed 38-48 grammes, and the
permanent acidity of the sap was very feeble. It was now set with the freshly divided
hase of the petiole in water, and on the following morning the weight was 28'72
grammes, and the permanent acidity of the sap intense.
Experiment lll.—k loaf of Kahw.hoe, w^eighing 30-58 grammes, and with highly acid
sap, was sot with the base of the petiole in water in absolute darkness at 10-30 A.M. At
1-45 P.M. the weight had increased to 31-08 grammes, and tho acidity of the sap remained
apparently unaltered.
Experiment /F.—Two leaves of Ealanchoe, one, a, weighing 29-6 grammes, the other,
23-4 grammes, were set with the bases of the petioles immersed in water. The sap in
both of them was highly acid in reaction ; « was exposed to direct simshine and h to
absolute darkness for more than three hours. At the close of this period « was slightly
Jimp, weighed only 27-7 grammes, and on section yielded comparatively little fluid, with
a mere trace of fixed acidity, whilst b was fullv turgid, weighed 23-71 grammes, and
yielded an abundance of intensely and permanently acid fluid.
of tlie LiDoein Societr, vol. XI. p. 213.