^ ^ THE CAUSES OF FLUCTUATIONS IN' I U R G E S C E X CE
demonstrate that, in both alike, the essential determinant lies in tlie relation between
supply and loss of water:—
Experiment /.—At 2-30 P.M. of a bright afternoon in November, the leaflets of four
plants of Cassia alata, growing in rather small pots, -wea-e all in almost complete nocturnal
position, whilst those of a plant of the same species, situated side by side with them
and under precisely the same conditions in regard to insolation, but growing in the gi-ouncl,
were in a state of full expansion. One of the pots was flooded with water, and an hour
later the leaves of the plant in it were almost fally expanded, whilst tliose of the other
pot-plants remained as before in the nocturnal position. At the same time the leaves of
an Emnthcmum growing beside the Cassias were very considerably wilted. Here there
were indices to the occurrence of general loss of turgescence in the Ermthmum leaves,
and of local loss of tiirgescence ia certam masses of tissue in the leaves of the Cassias
in pots, determined, in the latter at all events, simply by defective root-supply in relation
to transpiratory loss.
Experiment II.—At 1-30 P.M. on another day the leaves of two pot-plants of Cassia
alata had their pinnre in almost fúlly developed nocturnal position. One pot was flooded
with water, and at 2-50 P.M. the piunaj of the plant in it were almost fully expanded,
whilst those of the other remained as before.
Experiment ///.—Two pot-plants of Cassia alata were set side by side on a flat
masomy roof, so as to be fully exposed to the sun all day. At 1-20 P.M. of December
10th, one of them had its leaflets in the nocturnal position and the other its leaflets
nearly in the same state. The pot containing the former was flooded with water, and
three hours later the plant in it was fully expanded, whilst the other remained as before.
The following day was a cool, dry one, with only feeble sunsliine, owing to the general
clouding of the sky; but in spite of this both plants were in partial nocturnal position at
1-30 P.M., the development of the nocturnal position being much more advanced in the
plant in the pot which had not been flooded on the previous day than in the plant in
the other. The former plant was now flooded. Twenty-four hours later its leaflets showed
only a very slight tendency towards the assumption' of the nocturnal position, whilst
those of the other plant had to a great extent assumed it. The pot containing the
latter plant was flooded, and at 4-30 the leaflets were found fully expanded, whilst^those
of the other plant remained as before in the initial stages of the nocturnal position.
It would be easy to multiply the record of experiments of this nature indefinitely,
but, as tiie results were of a uniform natm-e, it would be superfluous to do so. Other
nyctitropic leaves just as clearly show the influence which is exerted on tiu-gescence by
variations in the relations between root-supply and transpiratory loss. In warm, relatively
dry weather, and under like exposure to sunshine, the leaves of pot-plants of Pithecolohium
saman will be found to be in the fully developed nocturnal position, whilst those of plants
rooted in the ground are in the fullest expansion. So again with the leaves of Cassia
sumatrana. Two plants growing side by sido and equally exposed to the sunsliine, hut
one rooted in the ground and the other in the crevices in a wall, day after day in the
afternoon during warm diy weather will respectively have their leaflets in full expansion .
and in the nocturnal position. In the above instances the most influential factor in
giving rise to diminished turgescence under otherwise favom-able circumstances was
I N TIIK MOTOR OEGANS OF L E A V E S . 39
insufficient root-su]5ply of water; but, of course, excessive transpiratory loss will produce
similar ejects, as the following experiment shows:—
Experiment IV.—Two leaves of Kalanchoe laciniata, each weighing exactly 10'38
grammes, were taken. One of them («} was placcd in a simple sealed chamber, and
the otlier (J) in one including capsules containing sulphuric acid and chloride of
calcium. The cliambers were placed side by side in front of a wmdow, so as to secure
their equal exposure to light. Twenty-four hours later, (c) remained quite plump and
rigid and weighed 10'2G grammes ; whilst (i) was flaccid throughout, slightly browned at
some point's on the margin, and weighed only 8-35 grammes. Both leaves were now
removed and planted with the bases of their petioles in moist earth, and here (5) shortly
regained its tm'gidity save in the browned areas. Here the influence of root-supply was
eliminated, as neither leaf liad any som-ce of sup^jly whatever, and the cxcess of loss in
weight shown by [h) as compared with (a) is thus to be credited solely to the action of
excessive transpiratory loss. The loss in weight in («) amounted to 0-13 and may be
taken to indicate the quantity of water required to saturate the air of the chamber
together with any expended in the course of assimilatoiy processes. This leaves a loss
of 1-91 in (5), ascribable to cxcess of transpiiutory loss determined by the aridity of the
air of the chamber.
It is the want of equilibrium between supply and loss that is the essential factor in
determining the result; not any absolute amount of cither. Tiie root-supply during the
course of the night must, as the soil cools, tend to diminish; and yet it secui-es the renewed
turgidity of tissues which have wilted during the day, because it no longer has to
contend with transpiratory loss. Once the dew-point has been arrived at, transpiratory
loss" comes to an end; and hence the continued supply, although not so great as dm-ing
the day, is able to restore tui-gesceuce in spite of the diminished manufacture of osmotic
products incident on absence of solar stimulation of the protoplasm. That it is the
cessation of transpiration, and not any other-nocturnal condition, which is the efficient
cause, is indicated by the fact that, on cloudy and dewless nights, recovery does not
occur, but that, apart from ineroaaed watei-supply, the wiltmg is carried on continually
into the next day. The same thing is, as we shall see when we come to consider
nyctitropic phenomena, very clearly indicated also by the fact that the nocturnal condition
in tissues exhibiting these attains its maximum in the early part of the nio-bt,
and then goes on gradually reverting, up to a certain point, towards the diurnal one!
In endeavouring to account for any particular fluctuations in turgescence and for the
effects followmg these, we liave to consider not merely the conditions of protoplasmic
activity regulating the manufacture of osmotic products, but also extrinsic conditions
affecting general supply and loss of fluid. The fullest stimulation of the protoplasm may be
present (as iu hot diy afternoons) coincidently with very imperfect turgescence, owing
to deficient supply and excessive transpiratory loss of fluid; on the other hand, a'high
degree of turgescence may accompany defective stimulation, as it does during the latter
part of dewy nights, because of the coincident continued supply and abohshed loss of
water. Tlie maximum of turgescence in tissues in which turgidity is dependent on continued
vitality is only reached where they are exposed to coincident maximum stimulation
of their protoplasts, to abundant supply and to abolished loss of water; but all minor
degrees may be present where these various factors are otherwise combined. In any
ordinary plant under normal circumstances ceaseless insensible fluctuations in turgescence