THE CAUSES OF PLUCTUATIOSS IX TUROESCESCE
or loss dosed when sneh stimulation is attondcd by doieotiTO root-supply or excessive
eraporativo loss of fluid, Wliilo the stimulation and functional actirity of the protoplasts
remain at a maximum, full turgoaceuco is impossible in tho absence of sufficient
water-supply; and, when the supply falls boneath a certain limit, the degres of
tm-gesconce in tho guard-cells is so lowered that they no longer possoss the excess of
functionally acquirod strength suiBdent to otereeme tho structural resistance of the
surrounding tissue; and, when once this condition has been reachod, closure of tho
orifices, or, in other words, an assumption of tho nocturnal position, must t a l e place.
The guard-colls are from their position and structure specially exposed to eTaporative
loss, and aro tboreforo specially liable to lose turgesconce, in spito of thoir functional
peeidianties, whenever tho rolations between root-supply and trauspivatory loss of
water are such as to imply a relativo excess in the latter. They are thus liable to
lose turgescence on diminished accoss of water to the roots, or on excessive evaporative
loss determined by depression of atmospheric humidity; and, as a matter of fact, wo
know that tho condition of tho stomatic orifices does v a r y in relation to fluctuations in
water-supply, quite apart from any fluctuations in degree of solar stimulation, and that
It IS prcdsely on their occm-rence that one of the main functions of tho stomata
depends. Tho movements determined by variations in the turgesconce of the guardcolls
aro sometimes connected with physiological and sometimes with pm-ely physical
causos, sometimes with variations hi the functional activity of the guai-d-cells, and at
others with vai-iations in the relations of general supply and loss of Suid dependent
on teUm-io and atmospheric conditions. In the one case we have the cell-sap becoming
less retentive of fluid, due to dhninished manufacture of osmotic products by the
protoplasts; in tho other, wo have a deficiency in tho amount of fluid which is
availahlo for retention, and in both cases the samo ultinate result is arrived at,
namely dimiuishod turgescence. Tho closure of tho stomata dm-ing the oourso of hot
dry days is thus the paralld of the wilting of common loaves and the assumption
of the noctm-nal position in nyctitropic ones under similar conditions. All three
phonomcna are duo to a loss in turgesconce, which is determined, not by any
dimmution in protoplasmic stimulation, but by deficient water-supply to satisfy the
osmotic capacities of tho products arismg in the coll-sap under tho inOuenee of solar
stimulation.
CHAPTER YI.
iinciitropic iiiobfinents of Itnbts.
I n the preTious chapter we have found grounds for believing that the movements
to which the opening and closui-e of the stomata are due depend primarily on
certain structural and functional differences between the guard-cells and the other
epidermal elements, and secondarHy on the relations between general loss and supply
of water determined by ateiospheric and tellm-ic conditions; and when we come to
deal with the movements of entii-e leaves, whether these be of a nyctitropic nature
or such as occur under conditions in which they ai-e ordinarily regarded as the result
I N T H E MOTOR O E G A N S OP L E A V E S. 47
of active contraction of the protoplasts of tho motor organs, we find abundant evidence
that they are due to an essentially similar causation. The movements in these cases
being determined not by individual cells, but by masses of tissue, are often of very
considerable magnitude and aro thus more readily appreciable than the stomatic movements
arc ; but, like the latter, they are detei-mincd eitlier by fluctuations in the retentive
power of tho cell-sap or by variations in the supply of fluid available for retention.
Accordingly we find tliem manifesting' themselves either as tho result of alterations in
stimulation of assimilatory activity of the protoplasts of the motor organs, of variations
in the relation of root-supply to transpiratory loss, or of variations in conditions affecting
filtration.
The periodic assumption of the nocturnal and diurnal positions by nyctitropic
leaves is a phenomenon of precisely the same nature as the coincident opening and
closure of the stomata and the changes in colour of the fronds of Selaginella serpens, in
being essentially due to alterations in the osmotic properties of the oell-sap, but the
action of these is affected by their coincidence with certain general relations of supply
and loss of fluid. Consequently, just as in SehgineUa wo find tho maximum of
whiteness in the fronds occurring in the first part of the night, so in the case of nyctitropic
leaves we find the nocturnal position attaining its maximum at tho samo time.
Tho attainment of the maximum of the nocturnal condition at or shortly after sunset,
and the occurrence of a reversion towards the diurnal one during the latter
part of the night, arc charactezistio fcatm-es in nyctitropic phenomena generally.
The maxiumm is attained at the time it is, because then the cessation of solar
stimulation of assimilatory activity and the consequent loss in osmotic property of
the cell-sap coincide with unultered, or inapijreciably altered, relations between general
loss and supply of water. Depression of turgescence reaches its normal maximum,
whilst absence of solar stimulation and continued transpiration are acting coincidently
to give rise to loss of fluid from the tissues; and, when one of the factors ceases
to act, a rise in turgescence naturally follows, and in certain cases may even go
so far as almost to re-establish the diurnal condition under normal circumstances.
During the day we have to deal with two opposing factors, increased protoplasmic
stimulation on the one hand, and increased transpiratory loss on the other. At
sunset wo have two factors, decreased protoplasmic stimulation and continued
evaporation, acting consentaneously to diminish turgescence. After the dew-point has
been reached, there is a continued absence of solar stimulation ; but tliis is no longer
associated with transpii-atory loss, and, the root-supply continuing as before, the cell-sap
is left at liberty to satisfy any osmotic capacities which it continues to have apart
from tho addition of special assimilatory products to it by tho protoplasm under the
influence of solar stimulation. Tho absence of solar stimulation does not cause a complote
cessation of protoplasmic activity or a total loss of osmotic property in the ccll-saj),
as death of the tissue does in those cases in which the osmotic property is dependent
on imstable compounds; it only gives rise to depression in them. Absence of solarstimulation
does not give riso to an abolition of functional activity ; it merely implies
withdrawal of ono special stimulus to its exercise. Even in the total absence of solar
stimulation, therefore, a great degree of turgescence persists under normal relations of
general supply and loss of fluid ; and, where supply continues and loss is arrested, the
turgescence may nearly attain the degree which it normally has when the tissues
are exposed to coincident solar stimulation and normal transpiratory loss, and may exceed