CAUSES OF FLUCTUATIONS TUB&ESCEKCE
tliilt co-existing with tliG fallest solai- stimulation and oicessive ttanspii-afory loss A
nycliti-opic leaf at any time ..f night is normally in a veiy difeorent condition from one
winch has loon immersed in boiling m t c r or exposed to the oontinned action of
the vapom- of chloroform. Maximum turgescence is only attained ,vhere full solar
stnnnlation co-exists with free supply and abolished loss of ivater; but all degrees of
turgescence short of this are constantly present in all ordinary tissues in relation to
the varying conditions of protoplasmic activity and general loss and supply of water
present in them under difeorent circumstances. A greater degree of turgescence will
naturally tend to be present where depression of functional activity coincides with
an absence of transpiratory loss than wl.ere the latter is still actively going on, and
hence we find the maximum departure from the diurnal state taking place at and
shortly after sunset. With the arrest of transpiratory loss the osmotic capacities of
the cell-sap are able fully to satisfy themselves. These, although not so great as
they are under the influence of solar stimulation, are yet sufficient to cause the turgescence
o£ the tissue to attain a higher degree than it attains under the fullest solar
stimulation where the latter co-exists with excessive transpiratory loss and insulîcicnt
root-supply. In the study of movements dependent on alterations in targescence,
whatever their nature be, it must always be borne in mind that we are dealing with
phenomena arising not as the result of the action of a single ultimate factor but of
the action ot various factors which ni£iy be associated with one another in very diSerent
combinations. Fluctuations in turgescence are capable of arising not only as the result
of fluctuations in osmotic property of the cell-sap, but also in conséquence of fluctuations
i a the relations between general supply and loss of water, or in the conditions o£
pressui-e, and therefore of filtrative loss to which the tissues are exposed.
All nyctitropic movements are in their origin essentially similar to the movements
detenniuing the opening and closure of the stomatic orifices. They are due to
the presence of masses of tissue differing from one another in structural and functional
îjroperty and so related to one another that they tend to give rise to displacements in
opposite directions. Theii- structural and functional differences give rise to the occurrence
of unlike fluctuations in their turgescence under the influence of like conditions
and therefore under the influence of varying conditions we find their relative strengths
varying, and movement taking place in one or other direction accordingly. lu^the
case of the stomatic movements, the motor apparatus—the guard-cells and the common
epidermal cells—ai-e in immediate relation to one another ; but, in cases where movements
of entire leaves or of large parts of leaves occur, the motor agents are generally
so disposed as to tell on opposite aspects of masses of passive tissue whose displacement
they make for. In the case of freely motile pulviui, for example, wo find the motor
apparatus consisting of tliick masses of cortical parenchyma, around a cord of passive
tissue consisting of modified wood and pith, or of the foiiner alone. The precise disposition
of the pads of cortical parenchyma in relation to the passive tissue on which
theyj act varies in different cases ; but we caa always recognise them as consisting of
two opposed sets, differing from one another ia stmctural and functional properties
and making for displacement of the passive tissue in opposite directions. In certain
eases the relation of the opposing tissues, however, more closely resembles that which
we find present in the case of the guard-cells and ordinary epidermal cells. The
movements in the opposite halves of the laminaj of the pinnas of many nyctitropic
leaves at a coi'tain stage of their evolution are effected under the influence of structural
IN THE MOTOK 01ÌGANS OF LEAVES. 4!)
arrangements of this type. At n very early stage of development the two halves of
the lamma have their upper smfaeos closely a|)pliod to one another, whilst in the mature
lamina) they are peimanently expanded in the same plane; but an intermediato staile
exists during which poriodic diurnal and nocturnal movements of alternate expansiL
nd ap2iroximation of the opposite halves take place. Here the
appearance of
opposing masses of active tissue telling
II intermediate passive one, but the opposdirect
ing tissues which deteiinine movemeiit a:
growth of parenchyma over the line of the midrib which ultimately gives l i e to pcrma"
nent expansion of the lamina by forcing out the divergent woody bmidles of the
veins from their original direction, and which in the intermediate stage gives rise to
periodic nyctitropi« movements of the two halves in relation to variations in the de^-ree
of tiirgcsccnce in the growing tissue under the iniluence of sunlight and darkness
All that is necessary in order to the occurrence of nyctitropic movements is tlw
presence of opposing tissue elements or masses of tissue, diflering from one anotiicr in
relation to one anotiicr. It is the
idergoiiig unlike fluctuations in turgescence under the
the
strucfure and function, and u
influence of particular condition
TIio difi'eroncos distinguishing the masses of tissue which make ro,spectively fc
diurnal and nocturnal position of entire leaves or large parts of leaves are of the same
nature as those which wo have already seen to distinguish the guard-cells and the
common epidermal elements. Tile tissues making for the diurnal position are relalively
late in development and rich in protoplasm, and specially in chlorophyll, as compared
with those which make for the noctmnal positiom But relative youth and
abundance of protoplasm à pm-i imply relative structural weakness and functional
strength; and the presence of chlorophyll implies a special assimilatory exercise of the
latter under the stimulant influence of sunlight. Under the latter a greater rise in
turgescence will thus take place in the masses of tissue which make for tho diurnal
position than in those making for the nocturnal one. During exposure to sunlight
tliere is a general rise in turgescence, but this is not uniformly distributed, but takes
place to a greater extent in one of the opposing masses of tissue than iu the other;
and tho foimor thus becoming temporarily tho stronger is able to dctomiine displacement
ot the parts to which it is opposed. Passive structural strength must, of course,
remain unaffected by exposure to light or darkness, but the strength arising from
functional soarces is not constantly alike, hut fluctuates with the degree ot turgescenoe
present at any given time. Any tissue in wlüeh an excess of tm-gescence arises in
with exposure to eertam conditions must become relatively stronger than a
vhich there is not an equal rise, tho difEcrence in strength being d i e c t ly
connectio
tissue in
related to the lUfTcrcnce in degree of turgescence. Under the influence of solar stinnilatmn
we have greater assimilatoiy activity in the tissues making for the diurnal than
for the nocturnal positi.m. But turgescence being directly related to assimilatory
activity, this implies a corresponding excess in strength aad a proportionate rise in
ciapaeity tor resisting the pressure of opposing tissues, and consequently displacement of
the latter occurs.
Apart from solar stimulation and general transpiratory loss, the position of the
various parts of nyctib-opio leaves will be determined by the relative .structural
strength and permanent functional activity of tho opposed tissues in tiie motor
apparatus. On the mcidence of sunshine, stmctm-al stength remains unohan..ed but
an excess of functional s t r c n g t h - a n excess of turgescence-arising in one of the opposed
tissues, displacement must occur in direct proportion to the excess. The amount of
. EOY. B o t . GAED. CALC; „ VI.