42 THE CAUSES OP FLUCTUATIONS IN TUEGESCENCE
protoplasts in an intermediate condition, not nearly so bigbly contractcd as they were
•when tbe plant was in the wbite condition, but at tbe samo time not so turgid as in
the diurnal green state, so that they do not fully occupy tbo cell-cavities and still leave
narrow peripheral spaces between their outer sm-faccs and the interior surfaces of tho
cell-walls.
We have here not merely an illustration of the chromatic effects which may be produced
by fluctuations in tm-gescence, but of the eSects of the latter in determining alterations
in bulk of protoplasts, and also of the fact that very considerable fluctuations in the
turgescence of the protoplasts of a tissue may occur without corresponding fluctuations
in turgescence of tho tissue as a whole. Whenever natm-al plasmolysis has occm-red in the
cclls, and the peripheral portions of the cell-cavitios have been occupied by air, tho
internal pvcssm-e oa the cell-walls must remain practically unaltered, however much the
protoplasts go on contracting and losing turgescence. The details which have been
ab-eady given regarding the structure of the fronds afford a ready explanation of the
exceptional phononiena occm-ring in the latter. Tho protoplasts of the superior epidermis
with thoir abundant chlorophyll content are specially liable to fluctuations in turgescence
connected with the variations in their functional activity—with tho variations in osmotic
properties in the ccll-sap related to the absence or presence of solar stimulation. Under
the influence of sunlight, assimilatory activity is stimulated; and, if this be associated
with properly regulated supply and loss of water, turgescence rises to a maximum. This
does not lead to any appreciable increase in volume of tho cells as a whole ; for changes
in thoh- bulk are rendered impossible owing to the fashion in which the tissue is stretched
over the resistent inferior epidei-mis and framed by the rigid margin of the frond, hut
it causes the protoplasts to fill the whole of the cell-cavities. On the diminution and
final entire cessation of solar stimulation assimilatory activity giudually falls to a minimum,
and at tho same time transpii'atoiy loss for a time goes on unchecked. There is a loss of
osmotic property in the cell-sap, whilst conditions of general loss and supply of water
remain practically unaltered, and with this a great fall of tm-gescence in the protoplasts
ensues. Owing to the unalterable dimensions of the cell-cavities tliis leads to a process of
natural plasmolysis, and the vacuities thus established are filled by air, the entrance of which
hito, as well as the escape of water from, the cavities being specially facilitated by the
peculiar pitted charactcr of the walls presenting to the free-surface. Presently, however,
with the fall of atmospheric temperatui-e, transpiratory loss diminishes, and, when the dewpoint
has been arrived at, ceases altogether, whilst root-supply continues, and now the
whitening of the tissue also begins to diminish steadily iintil the fronds appear almost
as they do during the day. So long as active transph-atory loss goes on, tm-gesccnce
of the protoplasts goes on falling below the level it would naturally sink to, simply
as the result of the loss of osmotic property connected with removal of solar stimulation,
and consequently the maximum of loss and of whitening of the tis.sue occurs.
On the cessation of evaporation the continued root-supply allows the ccll-sap to satisfy
i t s osmotic capacities unfettered, and hence an increase in turgescence takes place.
This is sufficient almost entirely to do away with the whitening of the tissue, but docs
not completely restore the proti Dplasts to their diunial stale of turgescence, which is
only regained when solar si-imulaiion of assimilatoiy activity once moi'e raises tlie
osmotic capacity of the cell-sap to a maximum. Precisely similar alterations in the turgescence
of the protoplasts, and therefore in their relations to the cell-walls of the ca\'itics iu
which they are conlainod, uiaj- be artificially iaiduced by exposing the fronds to the
IN THE MOTOE 0IIGA>'S OP LEAVES. 4:3
influence of chlorofoini vapour (plate I. fig, 10). The loss in turgescence Jiore, liowevcv,
is not accompanied by any such conspicuous change in colour as attends tho normal
nocturnal loss, as the fluid which escapes from tho interior of the ¡w-otoplasts into tiie
periphery of the cell-ca\-itics is not removed and replaced by air to auy appreciable
extent. In both eases, however, the essential phenomena are alike and consist in a loss
of osmotic property in the cell-sap, dependent on depression of the functional activity
of the protoplasts and determining a loss in tui-gesccnce. The transitory whitening of
tlie fronds occurring under normal conditions is quite distinct from tho discolom'ation
which occurs permanently in old fronds during continued periods of low temperatin-c or
excessive dryness. Tho change in colour under tho latter conditions is not of the same
dm-actor, being rather a yellowing than a wl.iteniiig, and is not connectod with loss in
turgesconco, but with deoolonrization in the chromatophoi-cs, a decoloarizatioa which is
apparently associated with increased formation of pennaiieut osmotic products in the
cell-sap, which serye to limit the range of fluctuations in turgescence. ï l i c normal
diurnal alterations in colour, on tho other hand, are essentially nyctitropic phenomena,
and are determined by the same primary factors as tho movements which wo have
next to consider.
In the case of Sela^huUa m-pms the changes of colour are dependent on movenrents,
but these are limited to the protoplasts, and are therefore imaccompanied by any
appreciable alterations in size of the cclls or displacement of masses of tissue. In the
ease of common nyctitropic movements alterations in turgescence take place in entire
tissue elements or tissues, not merely in protoplasts, and serve to give rise to movemonts
as distinguished from mere alterations in bulk or consistence, because thoy occur in
unlike amount in elements or masses of elements differing from one another both structurally
and functionally, and so arranged that any alterations in the bulk of one must tend
to produce corresponding alterations in the degree of resistance which it presents to the
action of the other. They are related to one another just as the scales in a delicate
balance arc; and just as airy alteration in tho weights in the former determines displacements
in the beam to wliich they are attached, so do any alterations in turgescence
affecting one of tho opposed masses of tissue to a greater extent than tho other determine
displacements in the structm-es to which they are related. In the case of common pulvinate
nj'ctitropic leaves which exhibit movements of elevation and depression, tho masses of
tissue on the upper and under sides of the passive, flexible, nnlignified vascular bundle
ai-e constantly tending respectively to depress and to elevate it. So long as conditions of
turgescence remain unaltered, or only fluctuate in like dii-ection and degree in both
simultaneously, no movement will occur ; but whenever we have either a rise or fall
in turgescence affcoting either of them in greater degree than tho other, displacements
proportionate to the difference in the resultant relative strength of the masses of tissue
must take place. Tho vascular bundle is in a position of unstable equilibrium, the
superior pulvmar pad constantly tending to depress it and the inferior one striving to
elevate it. The position of the bundle at any given time, in so far as local conditions
aro concerned, is dependent on tho relative strengths of the opposed pads, and any alterations
in theii- relative strengths must lead to con-esponding displacement.
Tho most universally diffused of all nyctitropic movements is that occumng in
the case of the stomata of the epidennal tissues, and as the essential features characteristic
of such movements as a class are in this instance presented to us in their
. Hoi. BOT, OAED, CALCVTTA YOL, Y I,