T'lE CAnSES o r FU7CTtr,l'nO.\-S IN TOBGESOESOE
condition of rolalivsly stable equilibrium ; but in the case of the primary jmlyinns
th,B 18 tsconntcd by the coincident increase in distal lorange, whereas in the
tortmry palvini it is either not interfered with or positively strengthened. It ean be
no wonder, then,'that a distnrbing factor should be capable of giving rise to effoots
difiermg in degree within the two ai-eas.
Phenomena of a Kto nature often manifest themselves in plants which are suddenly
transferred from a satm-atcd atmosphere to one of the same temperature, but considerably
drier. Under ciposare to the iniiueiice of an artiiciaUy saturated atmosphere in a
moist ehamber, if satEeient illumination be provided, the pinnules and secondary
rachises of the leaves assume their maximal diurnal position of extreme expansimi.
But the primary petioles, in place of tending to sink as they do under natural
ihiirnal conditions, become liighly elevated. All parts of the leaf at the same time pass
into a condition of abnoimally stable equilibrium, so that, as a rule, no movements
follow comparatively violent agitation. Bat, if the cover of the chamber be suddenly
and gently withdi-awa so that the tissues are ejposed to a relatively dry atmosphere,
movements occur after a brief interval. The,-.o, however, are fi-oquently confined to the
primary petioles, wliieh nndergo rapid and deep depression, and it is only occasionally
that any conspicuous rapid elevation of the pinnules manifests itself. On the theory
tliat movements are essentially due to active contraction of tlie protoplasts of the motor
organs, those phenomena must be taken to indicate that transfer from a- saturated to a
relatively dry atmosiihero causes protoplasmic stimulation more readily in the protoplasts
cf the primary pulvini than in those of the tertiary ones. Bat on the physical theory
no such arbiti-ary assumption is called for. When the distal portions of the loaves ai-e
saturated with moisture and have been continuously exposed to conditions favoming active
assimilation, theii- weight must attain its maximal development, and with this the strain
upon the primary pnlvinus must bo proportionately increased. Even in the absence of
saturation, prolonged exposure to coniKtions favouring assimilation causes sufficient
increase in loverage to aid matenaUy in inducing a depression of the loaves t o m their
late nocturnal position of extreme elevation ; but when transpiratory loss is absent, it is
unable to cause depression in spite of the aid wliich mere excess of distal fluid'must
yield to it. The strain, however, upon the p i i n a r y pnlvinus in maintaining erection
must be very great, and when the plant is suddenly exposed to conditions implying
an abrupt onset of transpii-atory loss disturbing thé pro-oxistent condition of hquid e q a it
brium, the effect natm-ully manifests itself most readily in it as the weakest part in the
motor apparatus. AVhere the atmospheric conditions are such as to give rise to transpiratory
loss in limited degico only, the disturbance tells only on the weakest part of the
apparatus, and depression of the primary petioles occurs; where the loss is greater
movements occur in the pinnules, but the loss must be very excessive to induce any
appreciable movements of convergence in the pinnoe, because in the socondary pulvini
conditions favouring a rapid loss in turgescence are present in much lower degree than
they are in the other motor organs.
The extent, rapidity and propagation of movements are then unequivocally related
tlii-eclly to the presence of conditions in tho environment regulating general supply and
loss of water. But if disturbances of the equihbrimn between supply and loss be the
essential factor which determines all movements not dependent on alterations in turgescence
connected with fluctuations in solai- stimulation and assimilatory activity, movements
ru THE MOTO'Il 0RG.1NS OP LEAVES. 103
ought to occur in connection with exposure to any conditions implying considerable
disturbance of pre-existing relations between supply and loss, and those ought to be
rapidly carried out where the cimditious are such as to implv rapid disturbance. It is
easy to acquire evidence that this theoi-etical requiiemont is amply met by actual
facts. The phenomena which have just been dcscribod as arising in connection with
the transfer of plants from saturated to relatively dry air are, of course, an example of
movements originating under such conditions, and one has not to look far for others of
a smiilar natm-e. It has already been pointed oat that when plants which have boon
for some time in the shade, and whose pinnules have attained a maximal degioe of
expansion are exposed to strong, direct sunshine, either as the result of the natural
dim-iial alterations in distribution of the latter, or of transfer from one locality to
another, gradual movements of elevation and convergence set in in the pinnules, so that
the leaves presently become mach less f u l l y expanded than they previously were. If
atmospheric humidity be v e i y low and the soil very dry, tho movements are continued
until the pinnules have assumed their maximal nocturnal position. If, on the other hand,
atmospheric and telluric aridity he not very great, the displacement is only a partial
one, ^ but it is always sufficient to cause the appearance of insolated plants to difl'cr
strikingly from shaded ones in then immediate vicinity. Whore atmospheric and telhiiie
humidity are alike moderate, the activity of root .supply is incapable of maintaining
active turgescence in tho tertiary pulvini at the same level as when transph-atory
loss was considerably less, bat it is capable of maintaining enough turgescence to
prevent the pmnales from passing on into the fully developed nocturnal position
But when atmospheric humidity falls very low, and when the supply of water in
the sod IS very small, the disproportion between root-supply and transpiratory loss
becomes excessive, and ' tho movements are carried out in maximal degree. The
phenomena arc precisely parallel to those occm-ring in the case of other nyctitropic
leaves under similar cicumstanccs, but the movements are more readily induced and
more rapidly eonduetod than they are in these because of the exceptional facilities for
transpn-atory loss and for rapid redistribution of liquid which the tissues of the
leaves present. Parallel phenomena of exceptionally rapid manifestation of the effects of
increased transpii-atory loss manifest themselves in connection with certain non-nyctitropie
leaves. An exposure to strong, direct sunshine, as brief or almost as brief as that
sufficing to give rise to sensible elevation of the pinnules of .Vimosa puaica wfll induce
perceptible general wilting in tho leaves of certain aroids, such as " P o i t o s mlacea" of
gardeners, unless both soil and aii- are loaded with moisture. In tho case <>{ '•'PoUm
violaceu" wo have evidence of general and evenly diffused loss in turgescence; in that
of Mimosa pudica of specially localised loss in turgescence as the result of exposure to
increased tninspii-atory loss ; and in both eases cases alike the loss in turgescence i
tionally rapid. No one would dream of ascribing the wilting of the leaves of
violacea'' to active contraction of tho protoplasts of the tissues; but the flinily fixed
belief in tho existence of special functional endowments in the protoplasts of the motor
organs of Mimosa, ptulica might lead to a belief that the parallel phenomena of the
movement of the pinnules was owing to some directly stimulant action of the sun's
» y s , wore it not that precisely similar ones manifest themselves wl.eie increased
transphatoiy loss arises quite independently of any alterations in conditions of illumination,
and that the results following exposure to direct sunshine vary
the extent to which facilities for transphatoiy loss are present.
s excep-
"Pothas